Of course, we know that there is much more to it than that. Jesus' command to the Apostles is to go out and teach as well as baptize "in the name of the Father, and of the Son, and of the Holy Spirit." Here we have God revealing Himself as a communion of three divine Persons. Here we have God sharing with us the truth about Himself, revealing to us His real identity.
In the words of Pope John Paul II, "God in His deepest mystery is not a solitude, but a family, since He has in Himself fatherhood, sonship, and the essence of the family, which is love."
We might ask ourselves: What difference does the mystery of the Trinity make? It makes a big difference.
For one thing, we cannot love what we do not know. We do not make complete gifts of ourselves to total strangers. (Ask someone to marry a complete stranger and see how well that works.) God allows us to know Him intimately as He is — one God in three divine Persons. He reveals Himself to us as a divine family. In fact, He goes even further. He loves us so much, He invites us to be a part of that family. Just last week we celebrated Pentecost, recalling how the Holy Spirit descended upon the Apostles in the Upper Room. In our Baptism, we too were sealed with the Spirit. By virtue of our Baptism we are brothers and sisters in Christ, and we can dare to call God "Abba" or Father. This means a Christian can never claim to be all alone in this world. We are part of a divine family.
Second, we are made in the image and likeness of God. We know that God is a communion of Persons. We who are made in His image and likeness are likewise made to be in relationship with God and with others. We are made to be communal beings. It is impossible for us to live in total isolation without doing some grave harm to our very being. We are made and meant to live for others. That is the nature of love and the nature of the God Who formed us in His image.
Finally, God is a family, a loving communion of Persons. We might look at a family, especially a large family, and sometimes see nothing but chaos, but we have to remember that a family is something sacred. A family is meant to be a communion of life and love modeled on the Trinity. It is in the family that we learn some important lessons about sharing, about being patient and forgiving. In the family, we become aware that we are always a part of a group larger than ourselves. There is no room for an arrogant individualism that can undermine family harmony. In other words, it is in the family that we learn how to practice a selfless, sacrificial love that is an image of the selfless love of the Trinity.
So what difference does one God in three Persons make? It makes a big difference. It tells us something about Who God is, and tells us something about ourselves, made in His image and likeness.
Fr. Grankauskas is parochial vicar at St. Mary of Sorrows Parish in Fairfax.
(This article courtesy of the Arlington Catholic Herald.)
Matthew 28:16.20 
Just before I ascended into Heaven, I summoned my Apostles to gather with me and receive my final instructions. As they saw me in my supernatural state, part human because of my physical appearance, and divine because I am God, they felt my Real Presence touching their hearts and souls, they felt the need to worship God. Some hesitated, because it is natural for human beings to use reason and create obstacles to their faith.