nature of revelation
I think the key point is "conceivable even though unimaginable”. We (mankind) could not have come up with a concept like "trinity" on our own-- there is nothing in our world that would cause us to come to this conclusion. However, just because it came via revelation does not mean we cannot apply our intellect and reason to it-- if we applied correct reason and came upon an irrefutable contradiction, then the concept *must* be false. But instead we see supporting threads that bear out the concept of trinity. It is a mystery we cannot fully comprehend, but, as you will get to later in the book, a "mystery" does not equate to something we simply accept as a premise, but rather , we have a limitless discovery that scales with our intellect and insight (Sheed uses an analogy of a candle in a huge dark room. The bigger and brighter the light we carry, the more we can see, yet for the "mystery"'there is *always* more to see and discover. This is one of the big AhHa moments for me in this book-- Mysteries are meant for us to continually explore. They are not a dead end.
Responses
There is a long history behind the doctrine of the Trinity. It is the intellectual product of the church. That does not exclude God but very much insists on the help of the Holy Spirit (as there is no church without the Holy Spirit.) As I still remember from school lectures, much discussion and controversy revolved around definitions. Even the term hypostatic union required immense efforts. The Cappadocian Fathers were accused of making imaginary distinctions. One of the Gregorys complained about his ceaseless efforts: I am mortifying my flesh in pursuit of truth, for what? At the very least it was an intellectual product involving the entire life of the saint...