The doctrine of the Holy Trinity is one of those complicated things that seems contradictory at first blush, then gets even more confusing the more you think about it. Bishop Barron offers a good explanation.
One way to think of it is when you picture yourself. You have an image of yourself in your mind, and you love yourself. But we don’t really know ourselves perfectly. When God “pictures” Himself, that image is so perfect, it’s as if it’s another Person, Jesus. And the love between themselves is so real, that love is a Person, too: the Holy Spirit.
Leo Trese puts it another way in The Faith Explained:
However, we do attribute to the individual divine Persons certain works, certain activities that seem most suitable to the particular relationship of this or that divine Person. For example, it is to God the Father that we attribute the work of creation, since we think of him as the “generator,” the instigator, the starter of things, the seat of the infinite power which God possesses.
Similarly, since God the Son is the knowledge or the wisdom of the Father, we ascribe to him the works of wisdom; it was he who came upon earth to make truth known to us, and to heal the breach between God and man.
Finally, since the Holy Spirit is infinite love, we appropriate to him the works of love, particularly the sanctification of souls, since sanctification results from the indwelling of God’s love within the soul.
God the Father is the Creator, God the Son is the Redeemer, God the Holy Spirit is the Sanctifier. And yet what One does, All do; where One is, All are.
Reading 1
Ex 34:4b-6, 8-9
Early in the morning Moses went up Mount Sinai as the LORD had commanded him, taking along the two stone tablets.
Having come down in a cloud, the LORD stood with Moses there and proclaimed his name, "LORD." Thus the LORD passed before him and cried out, "The LORD, the LORD, a merciful and gracious God, slow to anger and rich in kindness and fidelity."
Moses at once bowed down to the ground in worship. Then he said, "If I find favor with you, O Lord, do come along in our company. This is indeed a stiff-necked people; yet pardon our wickedness and sins, and receive us as your own."
At first, I assumed that this was Moses going up the mountain to get the Ten Commandments, but this is actually several chapters later. While he was up there, the people made the Golden Calf. Moses returned, saw the idol worship, threw the tablets on the ground, and all kinds of drama ensued.
This is after all of that. Moses finally calms down, and then what does he do? He goes right back to God. Carrying two blank stone tablets. He’s saying, “God, I don’t know what to do with these people. I don’t have any idea. Look, I brought literal blank slates. Tell me what to do.”
Responsorial Psalm
Dn 3:52, 53, 54, 55, 56
R. Glory and praise for ever!
Blessed are you, O Lord, the God of our fathers,
praiseworthy and exalted above all forever;
And blessed is your holy and glorious name,
praiseworthy and exalted above all for all ages.
R. Glory and praise for ever!
Blessed are you in the temple of your holy glory,
praiseworthy and glorious above all forever.
R. Glory and praise for ever!
Blessed are you on the throne of your kingdom,
praiseworthy and exalted above all forever.
R. Glory and praise for ever!
Blessed are you who look into the depths
from your throne upon the cherubim,
praiseworthy and exalted above all forever.
R. Glory and praise for ever!
This is the prayer that Shadrach, Meshach, and Abednego sing while they’re in the fiery furnace. When they say God is to be praised “forever,” they mean at all times, even the worst of times.
That’s primarily what Moses was angry about—the people worshipping a false god. Shadrach, Meshach, and Abednego, on the other hand, don’t give up even when they’re one fire.
Reading 2
2 Cor 13:11-13
Brothers and sisters, rejoice. Mend your ways, encourage one another, agree with one another, live in peace, and the God of love and peace will be with you. Greet one another with a holy kiss. All the holy ones greet you.
The grace of the Lord Jesus Christ and the love of God and the fellowship of the Holy Spirit be with all of you.
Paul’s Second Letter to the Corinthians was written in the fall of 57. Not ‘57, like it’s the middle of the Eisenhower administration. Just plain 57, as in a couple of decades after Jesus’ death and resurrection. Many of the original twelve apostles are still alive.
Even this early in Church history, we’ve got a clear, unambiguous statement of belief in the Holy Trinity. This isn’t some doctrine made up by theologians hundreds of years later; the very earliest Church fathers, who knew Jesus Christ personally, believed it.
Alleluia
Cf. Rv 1:8
R. Alleluia, alleluia.
Glory to the Father, the Son, and the Holy Spirit;
to God who is, who was, and who is to come.
R. Alleluia, alleluia.
This is probably not really theologically sound, but I do tend to think of the Father, Son, and Holy Spirit in terms of parts of the Bible. God the Father dominates the Old Testament; Jesus is obviously the protagonist of the Gospels; and Acts all the epistles are about the Holy Spirit working through the church.
Gospel
Jn 3:16-18
God so loved the world that he gave his only Son, so that everyone who believes in him might not perish but might have eternal life. For God did not send his Son into the world to condemn the world, but that the world might be saved through him. Whoever believes in him will not be condemned, but whoever does not believe has already been condemned, because he has not believed in the name of the only Son of God.
John, like Paul’s letters, was written early enough to indicate these beliefs about Jesus were pretty constant throughout early Church history. They never believed Jesus was just great teacher or good man. He’s the Son of God, sent here to save us from ourselves.
It’s a part of Christian theology that Jews didn’t anticipate. They never thought God would come in human form, but here we are.