The basic assumptions of a theory and empirical analysis of religious evolution include the following theses (1):
1. Religion evolves by differentiating itself from its environment and forms as a system.
1.1. No system exists without environment. Therefore, the religious system exists in difference to and at the same time in relation to its, i. e. religion-specific, environment.
1.1.1. Differentiation does not equate to separation or isolation, but means distinction with its components reciprocally determining each other.
1.1.2. Consequently, the religious system is no entirety that is composed of parts as perceived in the Aristotelian tradition. In fact, the structures and processes of the religious system—according to the newer systems theory (Luhmann 1975b, 194)—are only possible and comprehensible in relation to its environment. Generally, it is the religious system’s relation to the environment that determines what functions as an element, and what as the relation between elements, within the religious system.
1.1.3. The differentiation of the religious system is carried out by the externalization of its environment, to determine it religiously within the system.
1.2. The religious system has—as has any system—a system reference, which mediates the self-reference (system perspective) with the other-reference (system-internal observation of the system environment).
1.3. Furthermore, the religious system has—as has any system—a system-specific and a non-specific environment.
1.4. Seen from a sociological perspective, religion differentiates itself from society without becoming something other than a socio-cultural issue.
1.4.1. The religious system, therefore, stands in a threefold relation: to itself (reflection), to other societal subsystems (performance; German: Leistung), and to society (function) (Luhmann  1982, esp. 56).
1.4.2. The religious system identifies and distinguishes itself from other societal subsystems by the code transcendent/immanent.
1.4.3. The societal function of religion consists of ultimately coping with undetermined contingency by the means of distinguishing between transcendence and immanence.
1.4.4. The coordination between religion and other societal subsystems is, under the conditions of functional differentiation, neither a task of religion nor of other societal subsystems, but rather a matter of society.
1.5. The societal subsystem of religion stabilizes by internally differentiating itself.
1.5.1. The internal differentiation is, on the one hand, carried out segmentally by the evolution of various religious traditions, for instance, the so-called world religions (Stichweh 2001), which can be further distinguished as secondary religions from primary religions (Sundermeier/Assmann; cf. Diesel 2006). The internal segmental differentiation of the religious system generates internal boundaries. These are then strengthened by the contact between individual religions.
1.5.2. On the other hand, the religious system also differentiates itself in a socio-structural way alongside various social forms, which range from cults to groups, movements, schools, lineages, networks, and associations up to formal organizations.
1.6. The differentiation from the environment is stabilized by the external boundaries between religion and other societal subsystems such as politics, law, economy, science, health, social service, education, and art, all of which religion interacts with.
2. Differentiation can be best described by the means of the theory of evolution.
2.1. Evolution is to be formalized as “the accumulation of changes in the organization of successive systems, caused by the differential survival of replicating units of information” (Winter 1984, 68). It “behaves recursively, that is, applies the same procedure iteratively to its own results” (Luhmann 2012, 252).
2.2. Evolution proceeds in the increasing differentiation of the mechanisms of variation, selection and (re)stabilization (retention), but does not follow any inherent direction of progress (Ingold 1986, 14–28).
2.3. The increase of complexity of general evolution results from the differentiation between various evolutionary dimensions: the physical and the organic (plus the chemical dimension as a mediator between both dimensions), as well as the mental and the societal dimension (plus the cultural dimension as a mediator between both dimensions). None of the dimensions mentioned above can be traced back to one of the other dimensions, each rather has an emergent eigenstate. Evolution is a co-evolution to societal evolution, which, in turn, is a co-evolution to mental, organic, and physical Evolution.
2.5. The various kinds of evolution are associated through isomorphism.
3. Religion as a socio-cultural issue proceeds as self-referential communication.
3.1. Religion is not given a priori—whether that be metaphysical, ontological, epistemological, psychological, or neurophysiological, but primarily emerges and proceeds as communication. Something needs to be addressed as religion (including family-like terms or attributes in the tradition of Wittgenstein (2) to be identified as religion and to be distinguished from other issues—as an explicit attribution or implicit ascription (Taves 2009, 10). This applies to religious practice and even more to its scientific reconstruction.
3.1.1. According to the assumptions of the theory of evolution mentioned above, communication, including religious communication, is to be distinguished from mental, organic and physical processes as its environmental correlate. Consequently, humans are also a part of the environment of religion.
3.1.2. Religion can become thematic with a view to mental, organic and physical aspects, Humans can also be addressed, for instance, as acting or suffering beeings (or to put it in Max Weber’s terminology: as an “instrument” or as a “vessel”; Weber  1978, 546 et pass.). But in these cases as well, the subject is communication. In the case of addressing religion as a social issue, it is a matter of self-reference of societal communication, and in the case of addressing mental, organic, and physical environmental aspects of religion, it is a question of the other-reference of communication.
3.1.3. Of course, mental, organic and physical processes always belong to the conditions of religious communication in terms of necessary environmental correlates. However, if and—as the case may be—how religion system-internally addresses its environment, is not decided by the theory, but instead by religious communication itself—and within science: by empiricism.
3.2. Communication does not proceed following the model of transmission of information from a sender to a receiver. It is rather the unity of the distinction between information, utterance, and understanding and therefore a three-part selection process (Luhmann 1995c, 137–175).
3.2.1. Under the conditions of an indefinite amount of possibilities, communication has to be conceptualized as a selection carried out socially, limited in three different ways: An utterance occurs in contrast to another or remains undone—such as secrets or instruments of domination; information includes something by excluding something else; and understanding as the actualized difference of utterance and information is a selection in the sense that it can occur in many ways (something can be accepted, interpreted, rejected, misunderstood, etc.).
3.2.2. Utterance is only an offer to be selected, a stimulation that needs to be received to achieve communication.18.104.22.168. An utterance is most probably received as relevant to religion when performed in a context that is pragmatically— i. e., socially—determined as religious.
3.2.3. Information is not transmitted, but is rather generated as a result of an independent selection within communication.
22.214.171.124. Within religious communication, information is produced through the code of transcendent/immanent.
3.2.4. Only based on the distinction of message and information, communication may produce information as a difference from a difference (Bateson  1987, 276.321 et pass.): as the difference between one piece of information and another from the difference between utterance and information.
3.2.5. Understanding (in the sense of approval or rejection—including the possibility of misunderstanding) enables the fundamental distinction of message and information. As understanding is a constitutive moment for the occurrence of communication, communication is only possible as a self-referential process.
126.96.36.199. Understanding is religious if both a message is picked up as relevant to religion and information is produced via the code transcendent/immanent.
3.2.6. To understand communicative events as action (including responsibilities, intentions, and motifs), mental experience, or physical or organic states respectively processes, are communicative ascriptions.
188.8.131.52. Within religious communication, actions, experiences, and physical or organic states respectively processes can be determined as religious or as relevant to religion.
3.2.7. Every social system consists of communication. Within communication, selections with regard to the system are referred to as utterance, selections with regard to the environment observed within the system are referred to as information, and both types of selection are distinguished.
184.108.40.206. The religious system, too, consists of communication. Selections with regard to utterance are referred to as religious utterance, for instance as divination carried out by an expert, or as revelation from a god or a goddess that is conveyed by certain media, including prophets, scriptures, and special objects. Information selections are made on the basis of the code transcendent/immanent and refer to the religion-specific environment. The system-specific environment, which religion gains its semantic energy transfers it into system-relevant information, is therefore observed as specifically religious.
3.2.8. Information is generated within communication through coded events. Non-coded events come across as noise and can be perceived as a disturbance.
220.127.116.11. The more religious communication processes in a formalized way (ritualized, dogmatized, intensified through experience, etc.), the less unspecified noise is perceived. On the contrary, religious communication to a lesser degree of formalization is rather in the conditon to perceive noise and to treat it as irritation within the religious system.
3.3. In the study of religion informed by sociology, the distinction between information and utterance can be modeled as the difference between religiously determined semantic and social space.
3.3.1. The semantic space unfolds within the dimensions of cognition, experience, and materiality.
18.104.22.168. Religion, too, as a specific semantic space, unfolds in the dimensions of cognition, experience, and materiality.
22.214.171.124. While cognition refers to the production of knowledge (narratives, cosmologies, dogmas etc.), experiences are socially constructed schemes of mental perception (as a condition of meditation, mystical experience, etc.). Within the dimension of materiality, communication other-referentially bears upon organic as well as physical processes and states.
3.3.2. Social forms constitute the social space, including groups, movements, schools, lineages, networks, and associations, up to formal organizations. Social forms can be distinguished by the degree of formalization.
3.3.3. Semantic space and social space are reciprocally related to each other. The social space can always only be accessed semantically. A certain coherence of religious communication needs to be socially designated, for instance, as a ritual, an order, or a church. Conversely, the social space determines the respective configuration of the three dimensions of the semantic space. A religious group, for example, can accentuate the dimension of experience; a ritual process is more strongly related to the body and objects; and an interreligious dialogue might highlight the cognitive dimension. The social space is the entity of the regulated performance of semantics as a “supply of distinctions” (Stichweh 2006, 161).
3.3.4. The social space is nested in the societal space, which differentiates itself with segmentation, stratification, and function.
3.3.5. The history of religions can be studied with a focus on the mutual relation between religious semantics and religiously determined social space, as well as with regard to the societal structure.
3.4. Religious communication, as with any kind of communication, is based on the actualization of sign processes.
3.4.1. A sign is always three-part. It consists of the sign vehicle (also representamen) (firstness), the reference to an object (secondness), which itself has the form of a sign, and the interpretant, which mediates between the sign vehicles and the sign objects (thirdness).
3.4.2. The interplay of the three sign components ensures the interaction of presence (firstness), representation (secondness) and presentation (thirdness).
3.4.3. A sign is only a sign if it is translated into another sign and is further developed there (Peirce 1994, CP 5.594).
3.5. Systemic communication draws energy from the system-specific environment by means of analogies (metonymies and metaphors) and transforms it within the system to digital literality. Aided by metaphors from cellular biology, which, in turn, are taken from the metaphor of scripture, the following can be formulated: A communication system requires two processors for regulating the double input-output-process of transcription and translation.
3.5.1. This applies to all societal subsystems, including religion and science.
3.5.2. Religion and its scientific reconstruction are analogically related to each other, but do observe each other reciprocally in each case within the system in the mode of digital literality; in the case of religion based on the code transcendent/immanent, in science based on the code true/false.
4. In summary, religion is a societal communication system, which intrinsically emerges, reproduces, and further develops. Based on specific sign processes within societal differentiation, it is responsible for ultimately coping with undetermined contingency by the means of the code transcendent/immanent. Ultimately coping with undetermined contingency means that religion becomes effective, if other social processes—such as technology, law, insurances, politics, economics, medicine, etc.—have reached their limits in coping with contingency determined respectively. With the aid of analogies (metonymies and metaphors), religion gains semantic energy from its system-specific environment and, based on the transcendent/immanent code, transfers it to system-specific, digital Information.
The basic assumptions suggest how scientific knowledge on religion can be achieved by the triangulation of systems theory, the theory of evolution, and semiotically informed communication theory. These insights could not be achieved with only one of the three theories: the differentiation between religion and its environment can be reconstructed from an evolutionary perspective; the objcts of the theory of evolution can be understood as emerging systems; and the semiotically informed theory of communication clarifies the performance conditions of the combination of the theory of evolution and systems theory as well as its objects. Conversely, the combination of systems theory and the theory of evolution can describe how communication, including religion and science, evolves and structures itself by Differentiation.
In the following chapters, the preceding theses shall be explained to some extent. In the third chapter, I start with system-theoretical considerations on religion. The fourth chapter deals with questions regarding the theory of religious evolution. The fifth chapter focuses on communication-theoretical aspects. The sixth chapter broaches the issue of the relation between religious evolution (empiricism) and its scientific description (theory), and the seventh chapter contains thoughts on the reciprocal transcription of metaphors steming from the natural sciences and the social sciences as well as cultural studies.