Just to make it clear, when I say that Plantinga is irrational to believe in the trinity and hell, I mean that his reasons for believing these things are bad reasons. He believes in these things because they come from scripture and the Christian tradition, but there is absolutely no good reason to believe that the Bible is divinely inspired or that the early church fathers were divinely inspired when they came up with these doctrines.
And the same goes for things like the idea that Jesus was both fully human and fully divine at the same time. It’s not really necessary to go into the logic of it all and try to figure out whether this doctrine can somehow be made logically coherent. All I would say is that it’s irrational to believe something like this that you find in scripture or tradition in the first place. If we had good reasons for thinking that everything the Bible and the early church councils said was true, then OK maybe it would be rational to believe these doctrines, but we don’t have any good reasons. Plantinga believes these doctrines for exactly the same reasons that a fundamentalist Muslim believes their doctrines, and in both cases it’s irrational and should have no place in philosophy in my view.