This is silly. Just because something is ‘understandable’ [his word - I’d say ‘knowable’ or ‘cognizable’] in some respect doesn’t mean it’s understandable in a complete or total manner. I could know, say, that “an animal is in the doorway,” without simultaneously knowing that “a man in the doorway” or that “my friend, Ed, is in the doorway.” These all know the same thing, Ed. They just differ in the extent to which they know him. 

Additionally, and perhaps as a better example since we’re handling instances of reasoning to a conclusion, I could know the earth is a sphere by starting with the curvature of the horizon or I could go into space and observe it.

The point is even stronger when we understand that God is ‘incomprehensible,’ meaning His nature infinitely exceeds [in content, let’s say] the capacity of our knowing powers. Meaning we can perhaps know Him in a restricted manner, relative to our natural intellectual capacities, but never in such a way that we get a ‘total’ or ‘full’ grasp of Him.

(P.S. this last bit is why I don’t like the word ‘understandable,’ for this signifies a total apprehension [see Aristotle, Ethics VI], whereas you don’t have to totally and completely apprehend something to properly ‘know’ it)