I suspect if people were to write a good story involving a smackdown between an atheist and a Christian, that is without disrespecting either camp one would not only have to observe real life debates and arguments but also read up on each camp’s opinions and stories like Divine Comedy and ‘Dialogue Between A Priest and A Dying Man’.
The very last one could be offensive to others but I think it takes a more nuanced mind to handle it. Religion’s like a fire, it can enlighten people but can also harm them if used wrongly. If you want to argue from your point of view, have more wit to it to make it accessible to a lot more people.
For every Divine Comedy and Narnia, there’s going to be preachy fiction that’ll never be mainstream. So if you want to preach the message, you should still make it good enough to be accessible to the masses if you seriously intend to appeal to them at all.