The pastor having sex with the females of his congregation is a literal trope. So this happening is no a surprise at all. Some women respond to perceived power and godliness in a manner that is considered sinful in the very religion they profess. The some pastors take advantage of that fact, preaching one thing in the pulpit while doing the opposite outside of it.
That man better get a DNA test for that child, because right now he can't even be certain that the child is his. He can't believe what she says, because this very act demonstrates she is neither responsible nor trustworthy. So yea, he should divorce and make sure that child is actually his.