In his ‘Religion Within the Boundaries of Mere Reason,’ Kant seems to develop a trinitarian ontology. There are some interesting questions raised by this.

What is trinitarian ontology? It seems to me to be at least two things:
1) the ‘being’ of the Trinity
2) the ‘being of beings’ in light of the Trinity.

When Kant says that what God is in himself is a mystery surpassing all human concepts, I read (and have elsewhere argued for) him as being radically apophatic. A question then; can the Trinity ‘be’ in any other way than relationally?

As St Augustine had already pointed out, to claim to understand the God is a sure sign of not knowing Him. Kant implies something similar, and both solve this through relation.

But Kant adds that the relation of the Trinity is not physical (it would seem obvious), but moral. And morality is the key with Kant. So, what does this mean?

Because a God wants to be served morally, he is ‘qualified’ in three specifically different ways, and so it is fitting that there is a designation for each personality of one and the same being.

Without such a distinction, a ‘pure moral religion’ would run a risk of degenerating into an anthropomorphic servile faith.

Three mysteries are revealed to us through our own reason:
1) mystery of the claim of human beings to be citizens of an ethical state.
2) mystery of satisfaction: we are of ourselves corrupt.
3) mystery of election: why God grants grace to some and not others.

That is, in a sense, as far as reason goes - for we can ask the questions and confront mysteries such as these, but our limited reason doesn’t have the power to comprehend them. But regarding our conduct, Revelation is understandable by all, so it is a primarily moral religion.

For those interested in these sorts of questions, @NTOntologies is the place to go, full of fascinating themes.

Other experts in the field are @RyanHaecker @EduardFiedler @MasperoGiulio @johnmilbank3 and many more. Here is a discussion they organised last year.