249 From the beginning, the revealed truth of the Holy Trinity has been at the very root of the Church's living faith, principally by means of Baptism. It finds its expression in the rule of baptismal faith, formulated in the preaching, catechesis and prayer of the Church. Such formulations are already found in the apostolic writings, such as this salutation taken up in the Eucharistic liturgy: "The grace of the Lord Jesus Christ and the love of God and the fellowship of the Holy Spirit be with you all."

The dogma of the Holy Trinity

The Trinity is One. We do not confess three Gods, but one God in three persons, the "consubstantial Trinity". The divine persons do not share the one divinity among themselves but each of them is God whole and entire: "The Father is that which the Son is, the Son that which the Father is, the Father and the Son that which the Holy Spirit is, i.e. by nature one God."  In the words of the Fourth Lateran Council (1215), "Each of the persons is that supreme reality, viz., the divine substance, essence or nature."

The divine persons are really distinct from one another. "God is one but not solitary." "Father", "Son", "Holy Spirit" are not simply names designating modalities of the divine being, for they are really distinct from one another: "He is not the Father who is the Son, nor is the Son he who is the Father, nor is the Holy Spirit he who is the Father or the Son."  They are distinct from one another in their relations of origin: "It is the Father who generates, the Son who is begotten, and the Holy Spirit who proceeds." The divine Unity is Triune.

The divine persons are relative to one another. Because it does not divide the divine unity, the real distinction of the persons from one another resides solely in the relationships which relate them to one another: "In the relational names of the persons the Father is related to the Son, the Son to the Father, and the Holy Spirit to both. While they are called three persons in view of their relations, we believe in one nature or substance."  Indeed "everything (in them) is one where there is no opposition of relationship."  "Because of that unity the Father is wholly in the Son and wholly in the Holy Spirit; the Son is wholly in the Father and wholly in the Holy Spirit; the Holy Spirit is wholly in the Father and wholly in the Son."

Okay.  Let's be honest.  The Trinity is an article of faith. Everyone tries their best to conjure up an image of how the Holy Trinity works, but no matter what you come up with, it's not even close to the reality.  So why's that?  Well, because we're human, and because we're human, our intelligence is limited to things that are material and things we can reason out.  Except our reason is limited to what we sense in the world around us.  So when someone says, "hey, explain to me the Holy Trinity," we can only get so far.  

This is called the effect of having "finite" intelligence, whereas as God has "infinite" intelligence.  Because we can't understand the Godhead physically, because, well, He isn't physical (minus the Son Incarnate).  

So to the best of our abilities, we can understand the following, which points us in the right direction (but still doesn't explain it entirely)

1.   Neither the Father, Son, or Holy Spirit is a created being.  If someone says, "the Father made the Son" or the "Father and Son" existed before the Holy Spirit, anathema sit!  (that's Latin for "let him be anathema")

2.  The Father, Son, and Holy Spirit are separate "persons" in the One God.  If someone says, "I like the Father.  But the Holy Spirit?  Not so much", anathema sit!  It's the same God.  You either like them all or you don't like any of them.  

3.  The Father generates (note it's not 'past tense') the Son;  the Son loves the Father in return (as Jesus always does the Father's will); and the outflowing of that exchange is the Holy Spirit.  But it's not an order of events.  We like to compare it to a man and woman producing a child, but that's a series of events that implies a beginning and an end.  The Trinity has no beginning and has no end.  It's just happening always.  And it was always happening, always.  Forever.  So if someone says, "I wonder if God loves the Son more than the Holy Spirit," anathema sit!  There's no lesser or greater love among the three Persons.  

4.  The Father is 100% the Son and the Holy Spirit, the Son is 100% the Father and the Holy Spirit, the Holy Spirit is 100% the Father and the Son.  (You're welcome.)

So this is just scratching the surface.  Much greater minds have tried to dig deeper and deeper into the depths of the Divine Mind.  But don't fret, because the only that matters is this:

The Holy Trinity is God, and God loves you.