"Thomas Aquinas" writes:
To one who has faith, no explanation is necessary. To one without faith, no explanation is possible.
Using the Oxford definition of faith we have:
faith: strong belief in the doctrines of a religion, based on spiritual conviction rather than proof.
And applying the definition to the quote, we have:
To one who has "strong belief in the doctrines of a religion, based on spiritual conviction rather than proof", no explanation is necessary.
read that a few times... does it make sense to you?