One of the most significant things that Dr. Weinrich taught me is that the doctrine of the Holy Trinity is not simply a matter of divine trivia, which happens to be true but doesn't finally make much difference. Actually, I'd always been captivated by the Trinity (along with the two natures in Christ and the "real presence" of His Body and Blood in the Sacrament of the Altar), but I don't think I'd ever really considered what it means or why it's so essential. I suppose that when I thought about the assertion of the Athanasian Creed, that one has to hold this catholic faith in the Trinity in order to be saved, I understood that in terms of rational agreement and intellectual assent. As though one could wrap his head around the Holy Triune God and comprehend the incomprehensible Three-in-One! But Dr. Weinrich challenged me, and helped me, to appreciate the doctrine of the Holy Trinity far more profoundly, while at the same bowing before this great Mystery. It is the catholic faith, after all, "to worship one God in Trinity and Trinity in Unity," and not simply to speculate about such things.
The insight I have most appreciated and benefited from is this, that "God is Love" because He is the Holy Trinity of three divine Persons, the Father, Son and Holy Spirit. The Father loves the Son from all eternity, and the Son loves the Father from all eternity, in the personal Bond and perfect Unity of the Holy Spirit. It is for the sake of this divine, eternal Love that God created the heavens and the earth, and man in His Image, male and female. It is for the sake of this same divine, eternal Love that the Father sends His Son to redeem and save His fallen creation, and the Son goes willingly to His death upon the Cross. That's what grace is. It is the divine Love that flows eternally from the very heart of the Father for the Son, which He freely chooses to pour out upon us in Christ, that He might become the firstborn of many brethren. Because His love does not depend upon who we are or how we act, but is inherent in this permanent relationship of the divine Persons, there is nothing fickle or contingent about it; we can be absolutely sure and certain of His love for us in Christ.
One of my favorite early eucharistic rites is that of St. Basil the Great. It is a theological masterpiece, and a marvelous example of worshiping the One God in Trinity and Trinity in Unity. In the Preface of that rite, the Holy Triune God is praised for who He is, the Father, the Son and the Holy Spirit, and each of the three divine Persons is beautifully confessed in relation to the Others. This Preface leads into the Sanctus, in which the Church on earth is joined with the holy angels and all of heaven in worshiping the true and only God. From the Sanctus, then, St. Basil's eucharistic rite gives thanks to God by confessing what He has done for us in the economy of salvation. Here there is the Creation, the promise of the Gospel following the fall into sin, the sending of the Son, His Incarnation, Cross and Resurrection, and the bestowal of the Holy Spirit in the Church through the means of grace. It is by these works of God for our salvation, centered in Christ Jesus, and especially in His Body and His Blood, that the Father, Son and Holy Spirit are revealed to us.
Along with St. Basil's eucharistic rite, one of the best confessions of the Holy Trinity (notwithstanding the special signifiance of the three great Creeds) is the third part of Dr. Luther's "Great Confession Concerning Christ's Holy Supper." Similar to his later Smalcald Articles, it's a kind of theological last will and testament, in which he comprehensively sets forth the entire scope of the Christian faith and life. He goes about it by confessing what the Father, Son and Holy Spirit do for us, and give to us, in the divine works of Creation, Redemption and Sanctification, and how all of this is graciously bestowed upon us within the holy Christian Church in the preaching and administration of the Gospel-Word and Sacraments. What it all boils down to is this, that the Holy Triune God, in love, shares nothing less than Himself and His divine Life with us. St. Peter says that, too, in his second Epistle:
"Grace and peace be multiplied to you in the knowledge of God and of Jesus our Lord; seeing that His divine power has granted to us everything pertaining to life and godliness, through the true knowledge of Him who called us by His own glory and excellence. For by these He has granted to us His precious and magnificent promises, so that by them you may become partakers of the divine nature, having escaped the corruption that is in the world by lust" (2 Peter 1:2-4).