Odds of Getting In
There are more than 400,000 missionaries in the world. So it's not like becoming a rocket scientist or a professional sports player. On the one hand, anyone can technically become a "missionary," provided they have some sort of funding source and know enough about their religion not to be discounted as a total quack. On the other hand, if you want church sponsorship of some sort, you need to be a regular face at a church with a congregation large enough and wealthy enough to be able to sponsor you. And trusting enough to want to sponsor you. But in general, if you're really serious about becoming a missionary and you demonstrate that commitment over years and years of church and community involvement, it shouldn't be too hard to join their ranks.