In the first five centuries of the common era, the kiss was a distinctive and near-ubiquitous marker of Christianity. Christians kissed each other during prayer, Eucharist, baptism, and ordination and in connection with greeting, funerals, monastic vows, and martyrdom. As Michael Philip Penn shows, this ritual kiss played a key role in defining group membership and strengthening the social bond between the communal body and its individual members. "Kissing Christians" presents the first comprehensive study of the ritual kiss and how controversies surrounding it became part of larger debates regarding the internal structure of Christian communities and their relations with outsiders. Penn traces how Christian writers exalted those who kissed only fellow Christians, proclaimed that Jews did not have a kiss, prohibited exchanging the kiss with potential heretics, privileged the confessor's kiss, prohibited Christian men and women from kissing each other, and forbade laity from kissing clergy. The study also investigates connections between kissing and group cohesion, kissing practices and purity concerns, and how Christian leaders used the motif of the kiss of Judas to examine theological notions of loyalty, unity, forgiveness, hierarchy, and subversion.