It has happened to all of us. Some person with a so-called testimony akin to Paul's Damascus experience visits your church and tells you his story. The story is extremely well told, interspersed with tear-jerking sad stories and side-splitting funny anecdotes, with a smattering of heroics.
The person seems to be so nice that we just can't help ourselves liking and believing this person. If we don't believe him, then our subjective ideas of salvation to Christianity will not be confirmed. So, we believe, because it is so great that a satanist or a Muslim had come to Christ and now have such an awesome testimony for Christ! Surely, this is the person God had been waiting for to get those amazing in-roads among satanists and Muslims!