In religion, assimilation refers to the passive or active inclusion of persons or aspects of another religion as members or elements within a particular faith or belief system. It is similar in intent to inclusivism and pluralism and stands in distinction to exclusivism.
Assimilation includes the normal religious indoctrination of children by their parents, as well as the religious conversion of individuals from one faith to another. It is an important, if not the most important, form of cultural assimilation, around with other traditions, social customs, and even terms of language are centered.
An example of religious assimilation in historical religions is the syncretism practiced by ancient Greeks and Romans.
References
Religious assimilation Wikipedia(Text) CC BY-SA