Donald Rumsfeld, as US Secretary of Defense, once said:
“There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don’t know. But there are also unknown unknowns. There are things we don’t know we don’t know.”
How did he know?
In theology, to comprehend is to fully understand, and to apprehend is to partially understand. We can apprehend the Divine Nature; we can know and understand God to a certain limited extent. What we know is true, but it is not comprehensive. We can never fully know God, we cannot “comprehend” Him, because He is infinite.
The mysteries of the Faith (e.g. the Eucharist, the sacrifice of Christ on the Cross, grace/free will, and many others) can be apprehended but not comprehended.