What's more, did you catch the corollary at the end? We are destined to share in that exchange. More on that in a moment.

The "Who" of the Trinity is not only descriptive of God's being, but it also gives some indication as to "what" the Trinity does.

Some background: Christians have faith in this Three-in-One God because of God's own revelation to us. God not only created time but entered into time and spoke to us. The primary reason for this divine intervention is that God longed to reveal Himself to us, and did so in ever increasing degrees over the course of human history.

Think about it: This is a God who wants to be known. By us! And loved by us. The most amazing thing about the glory of God is that it is something that God wishes to share with us.

God is not afraid to seek us out and invite us to be joined to Him in everlasting glory. God is our origin and our end.

Reread the Preface above.

That little phrase from the Preface about God "ever to be adored" refers to would-be adorers—that would be us, His children, and all the saints in heaven—the ones who are invited into glory by being baptized in the Name of the Father, and the Son, and the Holy Spirit.

The Church, as Sheed says, assumes you might already have this knowledge.

But just in case you need a refresher, the Catechism of the Catholic Church outlines this relationship with the Blessed Trinity, and our true reason for being and our true destiny, in its very first paragraph:

God, infinitely perfect and blessed in himself, in a plan of sheer goodness freely created man to make him share in his own blessed life. For this reason, at every time and in every place, God draws close to man. He calls man to seek him, to know him, to love him with all his strength. He calls together all men, scattered and divided by sin, into the unity of his family, the Church. To accomplish this, when the fullness of time had come, God sent his Son as Redeemer and Savior. In his Son and through him, he invites men to become, in the Holy Spirit, his adopted children and thus heirs of his blessed life. (CCC, par. 1.)

As mentioned, God's inner workings revolve around an eternal exchange of love. God ceaselessly call us into relationship "at every time and in every place." This is a big part of the "what" that God does.

If the Trinity is an eternal exchange of love, then its primary action or raison d'être is all about relationships—between the members of the Trinity and with us. This exchange of love motivates and animates all that God creates and does. God's revelation of Himself and His ultimate plan of redemption flows from this love. For God cannot deny Who God is at His most intimate core.

Christians, baptized in the name of the Father, Son, and Holy Spirit, are destined to share in that eternal exchange of love. Baptism is both a future promise of heaven, and of the relationship shared with God even now.

Let's spell it out the way the Church does:

The ultimate end of the whole divine economy is the entry of God's creatures into the perfect unity of the Blessed Trinity. But even now we are called to be a dwelling for the Most Holy Trinity. (CCC, par. 260)