Following on from a post on holiday tattoos in Jerusalem I’ve come across an interesting reference to tattoos and conversion in Nabil Matar’s article ‘‘Turning Turk’: Conversion to Islam in English Renaissance Thought’, Durham University Journal, 1 (1994), pp. 33-50.
Matar talks about the phenomenon of Christians in the Ottoman Empire tattooing their children with a cross. This provided an indelible mark of faith as permanent as circumcision, and as the tattoo was commonly placed on the hand, arm or face, it was a visible reminder of the individual’s religious affiliation (pp. 38-39). Matar argues that the use of such bodily markers was designed to counteract the attractions and habits of the cultural and religious environment of the Ottoman Empire and prevent conversion to Islam.
After all, if an accidental repetition of the Islamic witness (‘there is no deity but God and Mohammad is his prophet’) could result in a conversion by a slip of the tongue (p. 36) Christians had to remain vigilant if they wished to preserve their faith. In this instance the tattooed body not only signals to others that it belongs to Christ, but the sign of the cross acts as a talisman protecting the bearer from harm.
Conversion Narratives in Early Modern Europe 
Juliet Fleming has written about the Renaissance tattoo, including pilgrimage tattoos (article of 1997 and book 2001). See also M. Gaudio, Engraving the Savage.