Romans 11:33-36 / Matthew 28:18-20
Going therefore, teach ye all nations, baptizing them in the name of the Father and of the Son and of the Holy Ghost, teaching them to observe all things whatsoever I have commanded you.
Why are we commanded by our Lord to teach and to baptize, rather than simply to teach. Surely if all things whatsoever Jesus Christ commanded are good, right, and reasonable, as indeed many non-believers assert to be the case, then sacramental initiation into the Body of Christ seems redundant at best, covetous for souls and minds at worst. This is the anxiety that even many Christians have about the missionary impulse. Surely, they opine, what Christ has taught, his message of love and of suffering for the sake of the beloved, and especially for the poor, is a credible message to all peoples, whether washed by the waters of the font or not. To attach following the commands of Christ to assent to a specific account of the divinity connects, on this view of things, two unrelated activities, the one having nothing to do with the other.
However, such a view is deeply mistaken, and forgets that at the heart of the whole Christian revelation, indeed at the heart of man's communication with God since Adam, is the invitation to know and partake in the interpersonal life of the one, true God, who is a Trinity of Persons, a unity in Trinity, a triune Unity. What this means, among other things, is that the truth of the Trinity is at the foundation of all that is. It is what philosophers today would call "properly basic." That is, there is no other truth, no other foundation, no other basis from which to judge and understand, to analyze and categorize, the Trinity. Rather, the Trinity is the one and only basis from which anything else has any meaning, indeed the sole foundation upon which anything else has any existence whatsoever.
This is why we say, in biblical language, that God beholdest the depths and sittest above the Cherubim. That is, there is nothing distant from him, whether the lowest or the highest, because he is the sole and absolute source of any and everything that is. Because the Trinity is the foundation of all without itself any foundation, the basis and norm of all reality which admits to no deeper or more fundamental category or truth, the revelation of the Trinity and partaking in the interpersonal divine relations which are the one, true God just is what it takes to observe all things whatsoever Jesus has commanded us.
In response to those who would think they could reduce the commandments of Jesus to a more fundamental level, something which would be more basic, more foundational than God himself, Paul directs his pointed questions: For who hath known the mind of the Lord? Or who hath been His counsellor? Or who hath first given to Him, and recompense shall be made him? There is no principle, no truth, no basic intuition which is prior to God, nothing which comes "first" before there is the Holy Trinity. To know this is to be open to any and all truth, to the depth of the riches of the wisdom and of the knowledge of God. To be closed to it, to seek some "deeper" truth which would make the Trinity plausible, into which we can fit this "account" of a "perfect being" is radically to be cut off from knowing the world as it really is.
This is why baptism, and the mystery of the sacramental life, is prerequisite to being open to the fundamental truth of all that is. To know reality we must know God, and there is no God other than the Trinity, nor could there be any other God. This is a truth of which we can be certain, but which, because it touches on the inner life of God himself, will forever elude even our best attempts to decode or analyze it. It is only when remade into new sorts of creatures, only when we have been transformed, not by our own power and insight, but by the working of the Holy Spirit, and only when that transformation is nothing other than sharing in the very Person of the Son of God, such that we will share with the Father even as he and Son are one, that we can, even here and now, have a glimpse of what the world, and more directly what our lives, are all about.
This is why we proclaim the Trinity today and every day, This is the deepest mystery of our faith. We are proud to profess it, and to make it known to the whole world: Blessed by God the Father, and the only-begotten Son of God, and also the Holy Ghost; because He hath shown His mercy to us.