It is well-known that in contrast to other religions, Judaism does not proselytize. In fact, when a non-Jewish person approaches a Jewish court and asks to convert, halakha requires the court to point out the difficulties experienced by the Jewish nation in today’s world, the commandments that he will now be obligated in and the punishments that he may be liable for should he transgress them. All this is done in an attempt to discourage converts, because – as Rabbi Helbo says – “Converts are as painful to the Jewish people as is sappahat (a leprous scab).”
This statement is one that clearly demands explanation.
Tosafot offer no less than four suggestions, among them:
Converts are not so knowledgeable regarding Jewish law, and other Jews may learn from them
All Jews are responsible for one another, and converts may err more easily
We are commanded to be especially sensitive to the needs of the convert, and it is difficult to fulfill that mitzva properly.
Other explanations are suggested, as well:
The Rambam suggests that we are afraid that a convert will revert to his original set of beliefs, and perhaps will influence others to join him.
Tosafot in Kiddushin (70b) point to a statement made by the Sages that the Jewish people were exiled so that they would interact with others who would be convinced of the truth of Judaism and convert. Thus, in a sense it is the converts who are the cause of the Jewish exile.
Another approach that appears in that Tosafot is quoted in the name of “Avraham HaGer” – Abraham the proselyte. He says that this Gemara should be understood to mean that since converts are meticulously careful in their fulfillment of the commandments – much more than other Jews – the comparison puts non-converts in a bad light, which is why the converts are viewed as painful for the Jewish people.