Belief in God is rational.
Everything has a cause. So unless there is a first cause, then you would have an infinite regress. And then nothing could exist. Therefore there must be a first cause. Therefore God, the first cause, exists. QED. (You could prove the first step a priori that everything has a cause by noting that nothing can come from nothing. This makes it a priori, not just an empirical observation).
"I think it would be extremely helpful and revolutionary if we could determine where in the continuum one gives way to the other. Or is there no continuum?"
I did not mention it in my answer to him but I think he is dealing with the issues brought up by Frege and later on people. That is a good deal of 20th century philosophy tried to deny the existence of the a priori. They tried to use Frege for that. But I do not know. I think Michael Huemer also gives a great account of lots of levels of a priori knowledge.
I am reluctant to go further than this because I am kind of on thin ice. I perceive some connection between Kant and Hegel which would be great as far as Shalom Sharabi and Isaac Luria are concerned. But I am afraid to go too far with this. [What I mean is my level in philosophy has not gone much further than what I learned in high school. People like Kelley Ross and Michael Huemer have done a lot of great thinking about these things.