I agree we are rational beings to an extent. We seem to possess abilities that exceed other animals as far as we know. And we can understand and predict the natural world to a significant degree. But the fact that we are able to do this doesn’t necessarily (or rationally!) mean that we have unlimited powers to understand the world around us or the nature of existence. There may very well be limits to what we can understand. In fact I would put it stronger than that, there are almost certainly limits on the extent we can accurately understand the world and its nature.
As humans we misunderstand and draw false inferences about the world all the time. Some people think they can sing when they can’t, or think they understand why someone did something when they don’t, or perceive a shape to be a shape it isn’t, or are convinced a short line is a long line because of the influence of others, and so on. The scope for misunderstanding and delusion in every day life is enormous. And if we can be mistaken about such mundane things, is it not reasonable to suppose there are things God can understand that we can’t understand? Like the nature of evil and the purpose of existence? It seems like a perfectly reasonable suggestion to me that God will know things that we cannot understand.
The Bible teaches that God is good but it also teaches that we cannot understand his nature and that we have no right to challenge his justice or goodness.