A section of Kant’s Conflict of Faculties lays out “Philosophical Principles of Scriptural Exegesis for Settling the Conflict” of faculties.
The first rule is that a scriptural text whose theoretical claims transcend reason and morality “may be interpreted in the interests of practical reason.” If it contradicts practical reason, it “must be interpreted in the interests of practical reason.”
The doctrine of the Trinity provides an example: “taken literally, [it] has no practical relevance at all, even if we think we understand it; and it is even more clearly irrelevant if we realize that it transcends all our concepts. Whether we are to worship three or ten persons in the Divinity makes no difference: the pupil will implicitly accept one as readily as the other because he has no concept at all of a number of persons in one God (hypostases), and still more so because this distinction can make no difference in his rules of conduct. On the other hand, if we read a moral meaning into this article of faith . . . it would no longer contain an inconsequential belief but an intelligible one that refers to our moral vocation.” Kant goes on to allegorize the incarnation, resurrection, and other Christian doctrines into secular tropologies.
In Religion Within the Boundaries of Mere Reason, Kant indicates how this can be done. The genuine article of faith is that God is Love, and this can be articulated in quasi-Trinitarian terms: “in Him we can revere the loving One (whose love is that of moral approbation of men so far as they measure up to His holy law) the Father; in Him also, so far as He reveals Himself in His all-inclusive idea, the archetype of humanity reared and beloved by Him, we can revere His Son; and finally, so far as He makes this approbation dependent upon men’s agreement with the condition of that approving love, and so reveals love as based upon wisdom, we can revere the Holy Ghost.”
Yet this doesn’t at all mean that “we should actually invoke Him in terms of this multiform personality (for to do so would suggest a diversity of entities, whereas He is ever but single); but we can call upon Him in the name of that object loved of Him, which He Himself esteems above all else, with which to enter into moral union is [our] desire and also [our] duty.”
Anything beyond this is simply a datum of “ecclesiastical” faith, and therefore useful only to distinguish one faith from another. Beyond the allegory, there is no philosophical weight to belief in the Trinity. The sheer difficulty of Trinitarian dogma underwrites priestly privilege: “Few men are in the position of being able to combine with this faith a concept [of the Trinity] which is clear and definite (open to no misinterpretation); and its exposition concerns, rather, teachers in their relation to one another (as philosophical and scholarly expositors of a Holy Book), that they may agree as to its interpretation, since not everything in it is suited to the common capacity of comprehension, nor to the needs of the present, and since a bare literal faith in it hurts rather than improves the truly religious disposition.” Nothing that helps the priests can be a good thing.