Ruini outlined three ways of access to God, three proofs of his existence, not theological but rational, and therefore able to be presented to all, not only to believers.
The first way departs from the evident fact "that there is something rather than nothing." The second moves from the observation that the universe can be known by man. The third is based on man's experience of a moral law within himself.
Read the whole thing.
(Via New Advent World Watch.)