What is the Trinity?
The simplest definition of the Trinity is “Three persons in one God” (CCC 253). Definitions are not much help with understanding the trinity as God is impossible to imagine.
So let's start with God the Father. We have already discussed that God is infinite. God the Father, being God is infinite. St John said that “God is Love” (1 John 4:8) and since love is a gift of self, The father is a pure infinite gift of self. God the Father is so giving that he doesn't just give stuff to others, but he gives his very self. This self gift of the Father is so powerful, it is another divine person - The Son. All that the Father is, is given to the Son and is what the Son is. The Son as God receives all that the Father is perfectly. This giving and receiving has no beginning or 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, consubstantial with The Father”
Since the Son is also God and love, He also gives all of himself back to the Father. The mutual self-giving between the Father and the Son is the Holy Spirit, The Third person of the Blessed Trinity. The Holy Spirit "proceeds from the Father and the Son" (Nicene Creed). The Holy Spirit is the Love between the Father and the Son.
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. As Catholics we believe in only one God. We are monotheistic (mono: one, theos: God). The definition we began with was only a very simple definition and doesn't include the giving (or relationship) between each person, which is one of love. There are three aspects to the Trinity, person, relationship and unity.