There is only one God. But God has shown us something amazing about Himself: He is three Persons, the Father, the Son and the Holy Spirit. We call this the Holy Trinity.
The Trinity is the most important mystery of our faith. A mystery is something true that is so big we can never understand it completely, like trying to fit the whole ocean into a bucket.
Each Person is fully God. The Father is God, the Son is God, and the Holy Spirit is God, yet there are not three gods but one God. The three Persons love one another perfectly.
A story is told that Saint Patrick used a shamrock, one little plant with three leaves, to help people think about the Trinity. No picture can fully explain God, but it helps us remember.
Jesus told His apostles to baptize “in the name of the Father and of the Son and of the Holy Spirit.” Every time we make the Sign of the Cross, we say these same words.