Was I really baptized if the person who baptized me turned out not to be a believer?
I think so. Say you have a man who is in the water with you, and you are a believer, and he is self-deceived and will come to the conclusion ten years later that he was a hypocrite--he leaves the ministry, he leaves his wife, and he leaves the whole thing, throws it all away, and proves never to have been a believer.
When at that moment, as a deceived unbeliever, he speaks the truth over you, the main thing is 1 Peter 3:21 which says, "Baptism saves you, not as the washing of water from the flesh, but as the appeal to God for a clear conscience."
So, insofar as baptism is from my heart an act of faith directed to God, appealing to him for cleansing and for justification and forgiveness, it saves. It's an expression of faith. That is not dependent on the authenticity of the man who holds your hand as you are put under the water.
I believe that what is effective is not the word spoken over you by the priest or the pastor but the authenticity of what's going from your heart up to God in the act of baptism.
So I would not get rebaptized. If I discovered that Eugene Lawrence, my pastor who baptized me when I was ten, forsook the faith and declared that he was never a believer, I wouldn't do it over again. It was my faith that was decisive, which is very different from the way some people view baptism.