What is the Trinity?
The simplest definition of the Trinity is “Three persons in one God” (CCC 253). Definitions sometimes don't help much with understanding what is really going on. So let's start at the start which in this case is with God the Father. We have already discussed that God is infinite. God the Father is infinite. He is pure giving, infinite giving. God the Father is so giving that he doesn't just give stuff to others, but he gives his very self, the deepest giving possible. This self gift of the Father is so powerful, it brings about another divine person - the Son. All that the Father is, is given to the Son and is what the Son is. The Son as another divine person receives all that the Father is, perfectly, totally, unconditionally and infinitely. This giving and receiving has no beginning nor end. Therefore it has no start in time, but is timeless. We express this giving of the Father in the Nicene Creed when we say:
I believe in one Lord Jesus Christ, the Only Begotten Son of God, born of the Father before all ages. God from God, Light from Light, true God from true God, begotten, not made
The Son also gives all of himself back to the Father, totally, unconditionally, infinitely. The mutual self-giving between the Father and the Son is so incredible it brings about another divine person, the Holy Spirit. The Holy Spirit "proceeds from the Father and the Son" (Nicene Creed). The Holy Spirit is the Love between the Father and the Son. The infinite mutual self-giving of the divine persons is ultimate fulfilment and so is infinite love, joy and happiness.
We are now ready to better explain the definition of the Trinity, "Three persons in one God." (CCC 253) We have already mentioned the names of the three persons, Father, Son and Holy Spirit, which we will explore in greater depth soon. As Catholics we believe in only one God. We are monotheistic (mono: one, theos: God). We will explore this soon as well. But the definition doesn't include a very important aspect, the giving (or relationship) between each person, which as we will see is one of love. There are three aspects to the Trinity, person, relationship and unity.