Cum enim aliquis ad probandam fidem inducit rationes quae non sunt cogentes, cedit in irrisionem infidelium.
"When someone tries to prove the faith by introducing arguments that make no sense, he falls into the mockery of unbelievers." (Summa Theologiae, Ia, 32, 1)
One thing I've come to believe is that many people don't believe in God simply because they have never been introduced to a concept of God to which a reasonable adult could consent. In the same way, I think a lot of us Christians are merely so on the moral or cultural level for the same reason. Or if we do believe in God, we are functional unitarians because we have never been given a portable, reasonable account of the Trinity, or we are vague theists rather than Christians because we have never heard a cogent account of the incarnation or the union of God and man in Christ.
When was the last time you came out of Mass on Trinity Sunday, having heard something mystical and not just mystifying on the Blessed Trinity? Or on the incarnation at Christmas? Or even the Resurrection at Easter? It always makes me so sad when I hear from priests that they don't know how to preach during the Easter season. If we don't know how to preach the central confession of our faith, what are we preaching about in the first place?