Exodus 12:43 says that a "ben neichar" would not be able to eat the Korbon Pesach (the Passover offering) when the Temple is rebuilt, placing this category of persons in the same category as non-Jews. According to the Sefer HaMitzvot, Negative Commandment 128, explains this this mitzvah with reference to Targum Onkelos who says that a "ben neichar" is "any Jew who has converted" (Targum Onkelos, Ex. 12:43), and the Mechilta which says "[t]he phrase, 'Any ben neichar,' refers to a Jew who has converted and worshipped idolatry" (Mechilta D'Rashbi). See also Hilchos Korban Pesach 9:7, where the Rambam explains the expression ben neichar (literally, "son of a stranger") as meaning, "one who serves a strange god."
Given the fact that Christian censors often changed or removed phrases and sentences they deemed offensive to Christians, I would not be surprised if Onkelos was more specific and applied the term to Jews who converted to Christianity in his day. Is there any proof of this? Can we know from other sources if Christians were forbidden to register for the Korbon Pesach?