If you listen to Fr. Robert Barron’s homily for this Sunday he starts by asserting that Trinity Sunday is known around the preaching community as a nightmare. At the same time it is a theologian’s delight.
The reason is that the Trinity is a difficult dogma of the Church to summarize in 10 minutes. Yet it is at the very foundation of our faith. It is truly a mystery.
The Catholic Encyclopedia defines “mystery” for us as follows:
The Vatican Council has explained the meaning to be attributed to the term mystery in theology. It lays down that a mystery is a truth which we are not merely incapable of discovering apart from Divine Revelation, but which, even when revealed, remains “hidden by the veil of faith and enveloped, so to speak, by a kind of darkness” (Constitution, “De fide. cath.”, iv). In other words, our understanding of it remains only partial, even after we have accepted it as part of the Divine message.( Catholic Encyclopedia)