Trinity Sunday isn’t just about making sure everyone’s not a heretic. (Though, even in that respect it does have some use!) Trinity Sunday is the culmination of the church year and serves as its fitting conclusion.
Trinity Sunday comes directly after the rehearsal of God’s great acts of redemption in Christ in Christmas, Good Friday, Easter, Ascension, and Pentecost. Every remaining Sunday in the church year is named after it—all Sundays from here on out are “the xth Sunday after Trinity” in the 1662 Book of Common Prayer.
Up to this point the church year has commemorated God’s great acts of redemption which he has accomplished in history by the Son in the Spirit. On this Sunday the church now turns her attention directly to God Himself—the God who has so acted.
By doing so, the church acknowledges that God’s Being is more ultimate than His works, while also acknowledging that the latter reveal the former. It is in the great acts of creation and redemption that the Triune God is revealed, and this God so revealed is the Ultimate Reality.
On Trinity Sunday the church confesses that her salvation is not self-referential—a mere escaping of eternal torments or attainment of endless carnal delights—but is directed toward the Triune God himself, Father, Son, and Spirit, “in the knowledge of whom standeth our eternal life.”
On Trinity Sunday the church looks back on the great acts of redemption already commemorated and names the God revealed in them, and she looks ahead to the completion of her salvation in the beatific vision—in the full and direct vision and enjoyment of God the Father, God the Son, and God the Holy Spirit, to all eternity.
Brandon puts it very well:
The Trinity isn’t just a knotty concept to scratch our heads at, affirm, and then move on. It is a Reality—The Reality—that we have been caught up in. The Trinity is our salvation.
I bind unto myself the name,
The strong name of the Trinity,
By invocation of the same,
The Three in One and One in Three,
Of whom all nature hath creation,
Eternal Father, Spirit, Word.
Praise to the Lord of my salvation!
Salvation is of Christ the Lord!