2,160. Someone Who Claims to Be a Convert

Hilchos Issurei Biah 13:9

Let’s say that a female convert conducts herself according to Jewish practices at all times, observing the laws of family purity, separating challah from her dough, etc., or a male convert follows Jewish practices, immersing after an emission and performing all mitzvos. Such people are considered “righteous converts” (geirei tzedek) even if there aren’t any witnesses to corroborate their claims. Nevertheless, if they want to marry Jews, we don’t permit it until they bring witnesses or immerse in front of us. This is because they have a legal presumption of being non-Jews.

Hilchos Issurei Biah 13:10

If a person claims that he was non-Jewish but he was converted by a court, he is taken at his word because he himself both created and resolved the doubt. This was only the case in Israel during the Talmudic era because everyone there was assumed to be Jewish; outside of Israel, one must bring proof of conversion. After he does so, he may marry Jewish. The Rambam opines that this law was enacted to protect the lineage.