In my experience, the Christians and secularists who are interested enough to have a debate about religion are the least likely to have their minds changed.
I used to debate with Christians on the Secular Web back in the late nineties. Many of the participants on both sides were of course interested enough not just to be there but to pack all kinds of research and details into their often long, wall-of-text posts. But I never saw anyone's mind changed on anything.
People talk past each other in debates. Only in a heart-to-heart conversation when the participants know each other well enough to be completely honest is it possible for one mind to affect the other. Otherwise, it's all for show and for reinforcing their beliefs. Maybe there are are exceptions here and there, but I think that's the pattern.
This "primer" I wrote isn't for the benefit of either of the fictional debaters. It's for the reader to figure out why such minds are so seldom changed.
If Christianity started off as a cult, as you suggest, and it still operates as a cult for the true-believers, their minds aren't going to change in a debate, because their religious beliefs are self-reinforcing delusions. The reasons for their religious beliefs are wholly emotional, experiential, and nonrational.
A good example of this is the recent "exchange" between Dillahunty and Ray Comfort. Comfort preached at him whereas Dillahunty tried to have a rational debate. Comfort couldn't debate because he presupposes that there's no such thing as an atheist, only a victim of demon possession who needs to hear the gospel preached at him to have a hope of salvation by a magical loosening of the demon's chains. There's no fruitful intellectual exchange to be had between such alien worldviews. Comfort and fundamentalists need to be in therapy. Of course, there are also mentally ill secularists.