The idea of ad hominem is this. A person says they believe something, and then give you a reason for believing it. Now, if they expect you to believe it because they said so, then who they are is important. But if they give you a reason, then you have to assess not them, but the reason they give for believing something. So, if someone offers a reason for rejecting the death penalty, it doesn't matter if, say, they are an inmate on death row. If they argue The focus shifts from them to the argument they offer. To focus back on the person when they have offered a reason for what they believe is to commit the "ad hominem" (to the man), fallacy.
William Lane Craig argues as follows:
1. Whatever begins to exist, must have a cause of its existence.
2. The universe began to exist.
Therefore, the universe has a cause of its existence.
Craig offers arguments in defense of each of the premises.
What that means, is that it is ad hominem to reject the argument by pointing out that he would believe in God, because of what he takes to be the testimony of the Holy Spirit, even if other arguments were bad. It is ad hominem to argue that he had an emotional conversion to Christianity. It is ad hominem to say that he wants to believe in God, so he will produce whatever arguments he needs in order to believe. Since he has given an argument, critiquing any thing other than the argument is irrelevant.