The question at stake in this paper is how to identify a Christian in the Roman Society. It is now widely admitted that Christians cannot be recognized through their clothes in Late Antiquity, except for those converted to asceticism. Christians rarely revealed their identity in texts, but we can analyse some signs likely to reveal one’s christianness or religious conversion. First of all, there exists a number of other outward signs that allows one to tell who is Christian. All Romans – pagans, Jews or Christians – sought protection by wearing phylacteries around their neck. As for the Christians, their amulets or phylacteries are connected to the cult of the saints, since reliquary phylacteries could serve as suspension capsules or containers for holy relics. Christian phylacteries were by manifold aspects a threat against relics privatization led by clerics as it has been demonstrated by Peter Brown. But all christians cannot wear relics around their neck either because of their cost, or because the events prevent them from doing so. Secondly, Christian identity can be retrieved from texts and inscriptions through visual signs, such as Christian symbols (ie cross, Chi Rho, christogram). Onomastic change due to conversion to Christianity are attested in the post-Constantinian period. Some new Christians may use additional cognomen to show their faith in Christ, such as martyrial names, theophoric names, or names proving a process of religious change. Finally, studying the social network of Romans may also reveal religious changes such as Christianness.