Brother Manes began his sermon last night, "If you expect to understand the Trinity after tonight's sermon, I'm afraid you'll be disappointed. In fact, no matter which parish in Sydney you go to today, you will still come out not understanding the Trinity."
While this is true, as Frank Sheed points out, God does not tell us things for no reason. And the doctrine of the Trinity is emphatically not something God had to reveal to mankind. Indeed, it is, in a sense, His most intimate secret. It is Who He is in Himself. To realise this fact is to reject both the option of throwing up one's arms and saying, "Well, its a mystery!" and leaving it at that, and the alternative temptation of treating the doctrine as some sort of esoteric mathematics that a few experts may be able to puzzle out one day. The doctrine of the Trinity has been revealed to the Church, and the Church is not peopled exclusively by theologians. It is, after all, catholic.
The Trinity is Who God is in His deepest essence. To have a relationship with God is to have a relationship with the Trinity. To look forward to spending eternity with God is to look forward to spending eternity with the Trinity. It behooves us to give at least some thought to this awesome reality God has revealed to us about Himself.
Perhaps the reluctance of many Christians to devote mental energy to the doctrine of the Trinity as such is the suspicion that it is an almost gnostic area of theology, obscure and impenetrable, a 'mystery' in the modern sense of something unknown and probably unknowable. This is a fallacy however.
As Karl Barth is at pains to point out in his Church Dogmatics, "The economic Trinity is identical to the theological Trinity." When most Christians think of the Trinity, they think of a concept, something abstract. This of course is wrong, as God is the most concrete of all things, and the Trinity is Who He is. But the theological concepts used by theologians who seek to derive from this doctrine what God intended us to know by revealing it to us must never be left in the abstract. God is concrete and He has acted in history. In the economy of salvation, God is revealed as Trinity (this is what Barth means by the economic Trinity). Trinitarian doctrine is not arbitrary, not some idea that exists apart from our spiritual lives or from God's dealings with mankind. On the contrary, God's trinitarian nature is revealed in these very things. In seeking to know Him, we must come to know Him as Who He is, and Who He is is Trinity.
Making the effort to do so illuminates the rest of our faith, sometimes in surprising ways. For example, the statement made by John that "God is love" could seem like a simple-minded platitude. The doctrine of the Trinity, however, shows that, far from being something to be confined to one of those insufferable corporate calendars full of mealy-mouthed statements of seeming wisdom, John's statement contains perhaps the deepest truth of God's nature. For God is in Himself a community of Persons, each giving totally of Themselves to Each Other, from all eternity, each animated by that thorough self-gift, receiving and giving back. That is the Being Who holds all in existence from moment to moment. That is Who God is, and the story of the Incarnation is the story of that love at the foundation of the universe going forth to heal mankind and draw him into Itself (or rather, Himself, or Themselves).
Nor does rejection of the fullness of this doctrine amount to the rejection of an unimportant abstract theological concept. The JW's, who have rejected the doctrine wholesale and regard it as abhorrent and even demonic, must necessarily believe that God has not saved us, but rather someone else has (in their case, St Michael the Archangel). That gives the lie to St John, implying God remained distant from mankind and sent one of His creatures to drag us out of the mud of the Fall. Alternatively, there is the atheist's retort when confronted with the Redemption that God engages in divine child abuse by sending His Son to be killed for our sins. The doctrine of the Trinity enables the believer to answer and say, "No, God Himself suffered and died for my sake," because the Son and the Father both possess the Divine Nature in its totality- both are fully God.
This explains why the Catholic Church still stands firm in affirming the filioque clause in the Creed, as against the Orthodox. It is important and vital to believe that the Holy Spirit proceeds from the Father and the Son. Diluting the doctrine of the Trinity always has implications, though they are not always visible at first.
In allowing our minds (and even, God forbid, our intellects) to dwell on the mystery of the trinitarian nature of our God, we come to know Him better, not just in His relation and dealings with us, but- what is more extraordinary- as He is in Himself; and thereby we give ourselves more reasons for loving Him and more reasons to look forward to spending eternity in His Presence.
Note: actually, Brother Manes' sermon was very good. His opening remarks just provided a handy jump-off point.