Three cheers for the Trinity! The theological arithmetic of 3=1 lies at the heart of what it means to be Christian.

JUST BECAUSE A THING IS ABSOLUTELY VITAL TO US DOESN'T mean we cherish it as such. Consider the fundamental importance of gravity, without which everything on this planet would simply float off into cold, inhospitable space. The laws of gravity are essential to everything you've got planned for the rest of the day, but chances are you won't give them a second thought.

In the same way, most of us don't think about the breathability of the air or the drinkability of the water until they're threatened, even though life would end in short order without both elements. In free societies our daily liberty gets no thanks, and in educated communities no one falls on their knees in gratitude for the ABCs. Those surrounded by love may forget that love is not a given. I'm forced to admit that I've lived for half a century with most of my teeth, but I have never sat down to dinner or flashed a smile and given thanks for bicuspids and molars.

Life's fundamentals can often be measured by how unthinkable life would be without them. Since we don't have to think about them, we mostly don't; otherwise, our night prayers would last until morning, as we thanked God for every element in the periodic table and every cell in our bodies. Instead, we reserve our thanksgiving for unexpected graces, and our intercessory prayer for things that malfunction or relationships that go awry. Meanwhile the fundamentals stay put and support us, well, fundamentally.

TAKE THE TRINITY, FOR INSTANCE. THIS IDEA IS THE CENTRAL mystery of Christianity. It's the uniquely Christian way of talking about God. Without the Trinity you still have a lot of good ideas--but the entire framework of the faith of the church collapses. Say goodbye to the Creed, to the divinity of Jesus, to most if not all of the sacraments, to resurrection from the dead and life everlasting, at least as the gospel defines them. And that's just skimming off the top. What lies beneath or even inside these formulas and doctrines would be a more critical loss than the outward shell of religion.

Let me propose three crucial interior elements of Christianity that are shaped by our understanding of the Trinity: forgiveness, self-sacrifice, and the unity of the church. Now some religious principles are nearly universal, but not these three. Belief in the existence of God, any God, is enough to supply you with a sense of divine justice, for example.

If there's a higher authority by any name, there's got to be a higher perspective by which we are critiqued. Sooner or later, a day of reckoning might well come from that source.

But forgiveness is a crazier notion. Divine law rationally leads to divine judgment, but not necessarily--or even logically--to mercy. Forgiveness demands a detour in logic, which is why many people--believers and atheists alike--refuse to go there. Reward and retribution fit neatly into the scheme of righteousness: If you're good, you get a benign or generous outcome, and if you're bad, you deserve nothing or even, perhaps, suffering. But to hold out the possibility that a person dedicated to badness might, as a result of a fateful hour of repentance, have the slate wiped clean and the outcome transformed is just plain awful to many people. Forgiveness is simply unreasonable!

Forgiveness is also a kingdom notion, a last-will-be-first concept that the reasonable will not readily accept. Why should the meek inherit the earth, virgins give birth, the dead rise, and sinners be forgiven? Because Jesus, who contains the full authority of God, proclaimed the coming of such a society by being the first to break earth's rules for heaven's sake. Jesus was not reasonable. He worked on the Sabbath, touched the unclean, befriended women, chose flawed disciples to be his friends and heirs, forgave his enemies, promised paradise to an insurrectionist (read: terrorist), and kept on doing these things, so it is said, even after they killed him. Without the Trinity, we lose the divine authority of Jesus to proclaim forgiveness as a preferred and even consecrated activity.

NEXT, THERE'S SELF-SACRIFICE, NOT TO BE CONFUSED WITH plain old sacrifice, which has been kicking around the religious world forever. Burn some grain, a few pigeons, a lamb or bull on big occasions--nothing special about that. A child, a woman, or a notorious enemy might be put to death for religious ends, as per local custom. It's all been done before. Priests of old stoked their ovens regularly. Abraham dragged his son up a hill once with the same intention.

But what kind of divinity takes a look at all the holocausts being offered in the name of heaven and chooses instead to sacrifice himself? It's uncanny and fearful, and we don't know what to do in light of it. A God who would surrender divinity to mortality, infinity to flesh, omnipotence to powerlessness, "accepting even death, death on a cross"--well, that's just overwhelming. It demonstrates in flesh-and-blood terms the extent and cost of God's commitment to our world and demands a response that is not measured out in pious teaspoons of devotion. Sometimes you read about a parent running back into a burning building to save a child. God did that for humanity. God does that every day. The Trinity is fundamental to Christianity because it's how we come to know God as forgiving and self-sacrificing.

Finally, the Trinity is important because of how it shapes our community as well. The unity of the church is precious, something Jesus talks about and prays about especially in the Gospel of John. We often interpret that unity according to the limited goal of keeping the institution together. In this sense, the Great Schism between the Roman and Orthodox traditions in the 11th century and the Protestant Reformation of the 16th century were terrible blows to church unity. Others will view the modernism crisis of the early 1900s and the loss of "common ground" after the Second Vatican Council in a similar light.

But church unity is not simply an institutional reality. More fundamental is this: We are the Body of Christ, one with Christ, who is one with God. The Holy Spirit of God dwells in us. Sharing in this everlasting life, we are essentially joined across the boundary of death with the whole community of saints. Now this is what I call unity: Heaven and earth are reconciled, divinity and humanity share a meal, the living and the dead are one community in faith--not to mention the dissolving of separations between gender, culture, and social status, as St. Paul was quick to spell out. If we allowed such an understanding to shape the world we live in, what a place it would be!

IS THE TRINITY A DOCTRINE WE CAN CHEERFULLY SHELVE, one for the religious specialist but, like gravity, not something most of us need to worry our pretty little heads about? While the Trinity may not get more than a quick nod from me in my daily Sign of the Cross, I might be more thoughtful about the church's annual focus on this teaching and its implications. Believing in God, after all, is not what makes us Christian. Believing that Jesus reveals the heart of God is more to the point. In turn, Jesus promises--and delivers--the Spirit, who remains to illuminate the way for us. The teaching on the Trinity is our gravity, and it holds the whole Christian story together. Even when it's the last thing on our minds.

ALICE CAMILLE, author of Invitation to Catholicism and coauthor with Paul Boudreau of The Forgiveness Book, both available from ACTA Publications.