As a broad term, “homophily” describes the phenomenon of those with similar traits and characteristics inclining to associate with one another. "Religious homophily" is more specific, suggesting that people largely have a predisposed attraction to those who share similar religious beliefs as their own and, thus, associate principally, if not exclusively, with only a certain kind of religious grouping.
According to one study in France, it is religion (not ethnicity, gender, age, or social class) that dominates as the most significant determiner of homophily. Not surprisingly, then, religious congregations tend to exhibit characteristics of lower diversity and higher social embeddedness. Research shows that religious homophily increases an individual’s perceived social support from the in-group.
Indeed, this explains why about one-third of American churchgoers say they derive all or most of their personal friendships from within only the confines of their particular place of worship. Associating with theologically similar people creates the perception that a person has an alliance of friendships instrumental to their psychological well-being. Religious homophily tends to exclude certain types of friendships while creating greater insulation from the outside world.