The Holy Trinity
The Holy Trinity is the greatest mystery of our faith. Because it belongs to the Divine nature, which is completely different from our human nature, it is truly beyond our comprehension. It is only through Divine Revelation that we know about the Holy Trinity.
Although this lack of understanding about who our God is may feel frustrating, it is what sets us apart. No other religion in the world has this uniqueness about their god. This mystery proves that Christianity is not a man-made religion. Our faith originates from Divine Revelation. No human mind can create or grasp that God is three consubstantial persons, one God in three divine persons.
The well-noted Catholic apologist Frank Sheed gave a very interesting explanation of the Most Holy Trinity: Thinking about our own human nature, he opens with the idea that each one of us exists, and because we are spiritual, we also have an idea of ourselves. We think about ourselves, reflect on ourselves, know ourselves. Human beings are the only creatures on earth who write diaries! This is similar to what happens in the relationship between God the Father and God the Son. God the Father is spiritual, able to know Himself. He has an idea of himself. But, unlike ours, His knowledge is limitless, that idea of Himself is perfect and perfectly complete. To be perfect, the idea, or the Word, must share in God's own existence; the Word must be a Divine Person. And so, God the Father, from all eternity, knowing Himself, engenders the Son, the perfect image of the Father. Consequently, because both the Father and the Son are infinitely good and beautiful, once they know each other, they also love each other. Even we, when we think about ourselves, love ourselves. We want the best for ourselves. We are glad that we exist. Yet God's love, like his knowledge, is unlimited, and so this love, too, must be so intense and so full that it shares fully in the Divine existence; this love is a Divine Person - the Holy Spirit. This is the mystery we profess each week when we affirm our belief in One God, the Father Almighty; the Son of God, who is "consubstantial [one in Being] with the Father, God from God, light from light, true God from true God;" and in the Holy Spirit, the Lord, the giver of life, who "with the Father and the Son is adored and glorified."
When asked by a priest friend how he should explain the Holy Trinity when preaching, the great 20th-century Catholic Theologian Father Karl Rahner, SJ, simply replied: “Don’t!”