There are people posting on this thread, claiming to be christian, that think salvation can come without faith in Jesus Christ. Some of them think that if unbelievers have never heard the gospel message, they will automatically go to heaven anyway...so, why would they want to have missionaries...it would mean that some would hear and reject, sending them to hell...if only they hadn't heard, all would have been ok. See?
I see your logical point, but if they are true believers, they must obey the Commission of Christ to go to all nations. I have had this discussion many times, and your logic is good, except that if we are obedient, we do what Jesus told us to do.