I think that in Kant there is a kind of skepticism about any kind of thinking outside the realm of "conditions of possible experience" which limits what we can reason about. It is not just that you see this in his system but also in his separate writings about Swedenborg. All together you get the idea that he accepts there are metaphysical realities but best not to reason about what there is no way of testing.
[In a later essay, he held that even thinking about theological questions can cause a person to go nuts. So it was not just an idea that he had that Reason when going into areas of unconditioned realities comes up with self contradictions but specially any person doing so would also end up insane.]
I think that this that Kant noticed about the effect of thinking about spiritual things [making people insane] must have been noticed by him in his daily conversations. He was a great "socializer" as is not well known.
[ Rav Nahman's belief in the limits of wisdom is one of his most famous doctrines [I think it is mainly in LeM vol II]. There is a place in the "Hashmatot" at the end of the LeM [not printed in all editions] where he says openly that to wisdom was set a limit so that it could not expand beyond that limit.]
It is well known in the Lithuanian Yeshiva world that thinking about spiritual things is discouraged. Simply one ought to learn Gemara in depth and that is that.
[In short the less there is of this, the better.]
Also I am not saying that Kant needs to be taken here without a grain of salt. He certainly has a point. However it was Hegel who felt the dinge an sich is possible to get to and I also happen to have a lot of respect for a mystic of the Middle Ages Rav Avraham Abulafia. His approach to Jesus is certainly an eye opener. His basic idea is that Jesus would be someone along the lines of Joseph in Egypt. He calls him the חותם של יום הששי which you would think would refer to Yoseph, i.e. Foundation of Emanation. So why or in what way does Jesus fill that same job description?]
However there are ways of testing: It is not as if all religious systems are beyond testing. Not exactly scientific testing in a microscope. But rather in terms of character. And of course even with that people have free will so you can not get an exact result. For example it is rather clear that Nazi doctrines had something to do with WWII. So sometimes results can indicate something about a set of doctrines. You can see this also in Dante. The different levels of hell all have to do with traits of character. And clearly I have a feeling that Rav Nahman was right about a lot of stuff and the Gra and Rav Shach. So I do attribute to them a kind of sixth sense--an immediate non intuitive knowledge [not sensed with the five sense--but perhaps still sensed?]
So you can tell something about a religious belief because of the general kind of people that follow it.