This is an important question, because if the membership criteria is too lax, as it is with Baha'is, what ends up happening is that people with proselytizer personality types end up converting their friends who are also proselytizers, and then before you know it the entire religion becomes a religion of proselytizers and they occupy every meeting with discussions of how to convert people, and then the people who actually care about the substance of the religion stop attending out of boredom. I don't think other types of converts are a problem - only the proselytizers, because they grow like a cancer.

At the moment I am having trouble thinking of a requirement that would keep the proselytizers out, short of banning conversion or forcing members to keep their religion a secret. The problem with a written test is that if at some point a proselytizer becomes the head of the religious community, he will rewrite the test so that only proselytizers will join. This is actually happening in the Baha'i community right now, where certain privileges are conditional upon completion of a sequence of courses in proselytism. I can't think of a good requirement that will keep out proselytizers, but I am sure their is one.