What would be the personal, legal and social impacts for a young unmarried man converting from Judaism to Christianity during the 13th or 14th centuries in a European country? Would this person be accepted more after the conversion, or would he be considered an outcast by both groups? What could he do that he could not do before if anything?
First, thanks for this question--I really enjoyed doing the reading for it. Curiously, my go-do Medieval European Jewish history texts talk plenty about Christian efforts to convert Jews (both forcibly and with other forms of inducement), but very little about what happened after this occurred (and what they do say is mostly about the Jewish reaction to these converts, rather than their role in society). I always like a question whose answer makes me run to the books!
The place of these converted Jews in the era you're talking would be very tenuous. Relations with the Jewish community would be strained at best, though this would vary quite a bit depending on time and place. Some converts did maintain some connections with their former co-religionists. Jews would generally hope for their return, though their treatment upon returning could vary too--some were required to undergo a "purification ritual", which seems to have been a popular innovation, not a ritual required by Jewish law. It seems that for these returning converts, social ostracism when returning was a serious problem. Jews also feared and vilified converts to Christianity, since they would sometimes make accusations about Jews to authorities, from the accurate (that Jewish liturgy contains a prayer for the destruction of heresy, which presumably includes Christianity, and may've been originally about it specifically) to the dubious (that certain prayers contain oblique references demeaning Jesus) to the patently false (blood libels). These accusations by converts could and did cause persecution of Jews.
Their treatment by the Christian community wasn't anything to write home about either. First, other Christians would think their conversion insincere. Many Christians thought of Jewishness as being indelible, that being baptized and confessing faith would never make a Jew a Christian. There was, in fact, a saying in Germany about this: "Wenn die Maus die Katze frisst/Wird der Jud ein rechte Christ!", meaning "when the mouse gobbles the cat, the Jew will accept Christ!" Converts were mocked with glib lines about how the only efficacious baptism for Jews was drowning.
Converts to Christianity couldn't necessarily expect to greatly benefit financially by converting, either. In England, Jews renounced their property upon converting. There was a government house providing shelter and a stipend for converts, but its occupants existence was meager. In the Mediterranean, converts occasionally had special taxes levied on them, just as Jews did. Many were reduced to begging, but that didn't always help--beggars claiming to be converted Jews became a bit of a "thing" in Europe.
However, those consequences could've been the best option financially. Records from England show that as royal taxes ruined the finances of the wealthier Jews, conversion increased among people from non-nuclear families. This indicates that as the charitable systems of Jewish communities collapsed, Jews in dire financial straits no longer had a Jewish safety net to support them, but may've only been able to survive on charity from Christian conversion. Even a meager converts' stipend would've been hugely important to someone completely penniless, and with nowhere else to turn. There were occasionally legal benefits to conversion, which would make it attractive--keeping property in a divorce, or avoiding prosecution were scenarios in which converting to Christianity would've been beneficial (though the first is inapplicable to your question).
As a consequence of these things, converted Jews would sometimes band together. That house for converted Jews in England sometimes housed converts' descendants. Elsewhere, converted Jews were identified as a discrete legal bloc. In Italy they were accused of practicing endogamy. The identity of being a "converted Jew" could stretch on for a few generations.
So, a Jewish convert to Christianity would've had it pretty tough. Rejected by the Jewish community they abandoned, but often not really accepted by the Christian mainstream, they were on the social fringe (much as they were as Jews). While there were legal advantages to converting, which would've been substantial in some cases, converting didn't necessarily clear one of all legal issues associated with their former status.
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