>>508460540 (OP)Jewish communities are matrilineal. They don't proselytize or convert. Being jewish is about blood as well as belief.
So the only way Judaism gains more adherents is to gain more jews.
To gain more jews, their women marry gentiles, thus bringing them into the community and having a bunch of "properly jewish" children. Also this is the only counter they have to inbreeding, unironically.
This is also why jewish people seem to be every ethnicity under the sun, because they take in outsiders strictly through marriage.