Video from The Thomistic Institute
"[I believe] in the Holy Ghost, the Lord and Giver of life, Who proceedeth from the Father and the Son, Who, together with the Father and the Son, is adored and glorified. How can we begin to think about the mystery of the Holy Trinity? The Trinity is the mystery of God's inner life and we only come to know about it by way of divine revelation. We know God is the Holy Trinity because God has spoken and told us so, because God has become human. By faith, we know that Jesus Christ is the son of God, eternally begotten of the father, God from God, light from light, true God from true God. And by faith, we come to know the father and the son and the Holy Spirit personally as God. Thinking about the inner life of the Trinity fortifies the Christian wayfarer in his journey toward union with the Triune God. The Triune God (Aquinas 101) - Fr. Thomas Joseph White, O.P." from video introduction.