I agree that some exposure is necessary; however, the degree of exposure necessary for conversion to be possible is nowhere near the degree of exposure involved here. At one point I didn't know that there are infinitely many prime numbers, and I had to be exposed to it before I would believe it (since I don't generally go around trying to prove random mathematical statements); but I didn't have to be exposed to months of lectures on the subject or be surrounded by people who made belief in that proposition a cornerstone of social interaction with them.

In the case of missionary work, I'd point out that one reason for missionary work is to force the religion's members to publicly commit to and sacrifice for the religion. It's a type of psychological pressure on believers to make them do things that keep them within the fold, not mainly a way of gaining converts.