You claim you were a Christian once, but you never really experienced God and Jesus. If you had, you would never have become an atheist.
Many myths which religious theists generally, and Christians in particular, hold about atheists depend on the assumption that atheists don't really understand religion. When it is pointed out that most atheists used to be religious believers, people attempt to salvage their myths by arguing that the atheist was never a "real" believer. Christians, for example, will claim that no real Christian, having personally experienced God and Jesus, would ever leave Christianity.
To begin with, this myth commits the fallacy of Begging the Question because it assumes that there is a real God and Jesus to experience but that's precisely the point which is being debated. A Christian cannot prove that an atheist was never a "real" Christian and is therefore unreasonable in not believing in any gods by assuming the truth of their claim that their god exists. They have to be able to support their claims by relying on independent and mutually accepted premises.
If a Christian only accepts someone has having "spiritual" or "faith" knowledge, and thus as being credible, if that person "knows" God and therefore believes that the Christian god exists, then what they're saying is that they only consider someone credible if that person already agrees with Christianity. Would they accept it if an atheist said "you're not credible if you believe in superstitious nonsense like gods"? Of course not. Why? Because it's rude, arrogant, and a fallacy to demand that someone agree with you before accepting that they might have something credible, reasonable or justified to say on a subject.
Such an atheist would be saying that they have nothing to learn from Christians, that they already know all they need, and that they aren't interested in the possibility of seeing something new. Well, a theist is basically saying the same in the reverse situation. I'd say: the person who rejects all ideas except those which already agree with them is the one who isn't credible because they act like they are afraid of hearing something uncomfortable, or perhaps learning something new.
Second, the Christian here cannot presume to know what the atheist did or did not experience in the past because it's almost certain that they didn't know them back then. It's arrogant to presume to judge a stranger like that. In fact the atheist might have had some very intense experiences while a Christian, experiences which at the time they treated as religious experience.
Atheists don't necessarily dispute that religious believers have intense, important, and even-life changing experiences. What they will dispute, as skeptics, is the alleged source of these experiences. It's one thing to take a person at face-value when they report that they had a significant experience, but it's quite another to simply take at face value the claim that this experience was caused by God and was, moreover, an attempt by God to communicate with them personally.
What arguments or evidence, without resorting to question begging, can a person use to claim that whatever they experienced is necessarily that of the god they believe in? Perceptual recognition is something which can merit skepticism even in mundane matters we encounter in everyday life. Consider how easy it can be to make an error in recognition when it comes to the voices or faces or writing styles of people we know very well but how would we know the voice or face or speaking style of god?
Michael Martin offers the example of a friend claiming to have spoken on the phone with "someone who seemed to be the strongest man of County Cork." How on earth could such an identification be made merely on the basis of a voice? Perhaps if the person was an expert on Irish accents at least a small part of the claim could be justified but only a very small part. Most people would, at the very least, treat their friend's claim with a great deal of skepticism and would probably refuse to believe it.
These same problems occur with the claims made that someone has spoken with God or even just experienced God. This claim cannot be taken at face value: we need to know what part of this experience justifies the conclusion that it involved God with all of the qualities and attributes alleged for this god, like omniscience, omnipotence, omnibenevolence, etc. and not an experience of something else, even if it is another supernatural being.
We have every right to be just as skeptical of this kind of claim as we would about our friend's claim to have spoken on the phone with someone who "seemed" to be the strongest man of County Cork. Indeed, we probably have more reason to be skeptical here because at least we all accept that there is a County Cork, that men live there, and that one of those men can probably be judged to be the strongest of the group. We don't all agree that anything like supernatural beings at all exist, much less one like the god claimed by the theist who is reporting a religious experience.
A traditional question based upon this dilemma is, Are you so sure that you cant be fooled and it wasnt Satan who spoke to you? You dont have to be a believer in God or Satan to recognize the importance of this: no one has offered a sound basis for differentiating between an experience of a god and of something else entirely, especially the possibility of being fooled by some other being. The possibility of being fooled is no less plausible than the possibility of the report being exactly correct; the possibility of simply being mistaken is surely much higher. This means that anyone who has had a religious experience and expect others to take them seriously must be able to show how and why their experience can be differentiated from error on their part.