Reason is better than assumption.
The irony here isn't that people should be reasonable—that's pretty standard. The irony is what most people call their "religious belief" is actually just an excuse not to think deeply for one's self. Reason is far better suited for the task of understanding our lives.
Pure reason is bodily.
One ironic aspect of Kant's philosophy is that Kant views the body and the mind as a continual process. He explains that the ability to understand arguments and conclusions is essentially a manifestation of the universe's observability and logical order. This means that our physical bodies, our physical brains too for that matter, are expressions of the universe, even though the soul seems to be removed from the world somehow. Therefore the process of reason is continuous with the process of bodily existence.
Pure reason without experience is dangerous.
It's easy to draw arguments in theory, and it's especially easy to pretend to be reasonable when being self-serving. So Kant notices that pure reason is extremely rare en masse. He also notices that when people attempt to be reasonable about things that are transcendent, that their reason fails, because of its limited scope. And without being tethered to real experience, the idea of reason is not helpful. In other words, we must be honest and not argue for things we know to be false.
Paradoxes and the transcendent.
One main ironic component of philosophy itself (since at least the pre-Socratic Greek philosophers) is that paradoxical statements of seemingly contradictory natures can be simultaneously true. However, Kant observes that when this happens, often it's the result of the mistreatment of transcendent ideas. For instance, to say whether time is infinite or not is not paradoxical—it merely transcends the scope of human knowledge or reason. As an infinite phenomenon, a limited being won't be able to rationalize it.
The unsettling truth.
Instead of saying, "Look, we can throw the Bible away and be happier," Kant suggests that perhaps religion is deeply important and meaningful, as a way of making sense of the soul morally, but as a philosophy, it turns out that the truth is far, far more formidable than the superstitious religion of conservative people. To be objective is to risk destroying one's world view, to which Kant basically says, "Great. If it's not reasonable, don't believe it." It's better, in Kant's view, to be confused than to be wrong.
