That three persons are each fully God yet each the same God as the others is the central mystery of our faith. Indeed, the Trinity is the reality and the article of faith that I most love. It explains both why God is intelligible as love and why he is incomprehensible. If God is triune as the Church teaches, then his triunity is the model for all of creation and hence cannot be adequately understood from within creation; for the imitator and the lower cannot cannot give an adequate account of the imitated and the higher. But the Trinity is all the same the reality into which we are reborn as Christians. It is the reality into which God desires to draw all people, for he “desires that all be saved and come to knowledge of the truth” (1 Timothy 2:4). It’s a very difficult thing to talk about or even preach on because it seems so abstract. But I think something can be said to make it much more concrete and relevant.
Consider the structure of the ancient baptismal formula known as "The Apostles' Creed." I insert the text here for convenience:
I believe in God, the Father almighty,
creator of heaven and earth;
I believe in Jesus Christ His only Son our Lord;
He was conceived by the power of the Holy Spirit,
and born of the Virgin Mary,
He suffered under Pontius Pilate,
was crucified, died, and was buried.
He descended to the dead.
On the third day He rose again.
He ascended into heaven,
and is seated at the right hand of the Father.
He will come again to judge the living and the dead.
I believe in the Holy Spirit,
the holy catholic Church,
the communion of saints,
the forgiveness of sins,
the resurrection of the body,
and the life everlasting. AMEN.
Jesus comes forth from the Father and comes down to us. He takes flesh, suffers, dies, is buried, and descends even to Hades; he then rises from the dead and ascends into heaven. This is what St. Thomas Aquinas, following the Pseudo-Dionysius, called the exitus and reditus of God. Creation itself had come forth from God in a free choice of love and finds its goal in God, whose Holy Spirit vivifies and orders it. When it fell from that plan through the fall of Lucifer and the human race he seduced, God himself had to recapitulate the necessary movement by descending into the depths of creation so that he could elevate it by his victory and ascension. The exitus-reditus model is that of descent and ascent. The fullness of glory is thus attained by self-emptying for others who do not merit it: a process of pure love. Such is the dynamic model of all reality, both divine and created. In and through us, the reality of the divine love makes that of creation both possible and actual.
Such, then, is the structure of divine revelation itself. How God thus reveals himself also tells us what God in himself is like: a communion of persons who love one another by giving themselves to, and thus emptying themselves into, each other. The eternity and changelessness of God is thus dynamic not static. It is that of love. But it takes no time to be complete in God himself. It only takes time to be expressed and revealed in creation. That's what Jesus Christ, the incarnation of God the Son, was and is about. By doing in time and flesh what he eternally does in the Godhead and in spirit, the Son made it possible for us to participate in the very life of God. To avail ourselves of that staggering opportunity, we must agree to be reborn "by water and the Holy Spirit," becoming part of his Body, the Church, so that he does in us what he did once for all in his Pasch. Thus we are to love as he did, which can happen only because we are loved as he is loved eternally by the Father.
Properly appreciated, therefore, the doctrine of the Trinity tells us what life is for. The doctrines of the Incarnation, Passion, and Resurrection of Christ tells us how God made it possible for us to attain the goal. For motivation's sake, it's always best to keep that big picture in view.