In the study of philosophy, there are a couple of concepts called formal principle and material principle. The formal principle refers to the ultimate source or rule used to define a belief or philosophical system. The material principle refers to the core teaching or governing doctrine of a belief or philosophical system. The terms and concepts not only apply to philosophy, but also to linguistics and theology.

In Lutheran theology, the formal principle is most simply put as Scripture alone. The implication there is that other theologies use other or additional sources to define their doctrine. For instance, in Roman Catholic theology, the formal principle is Scripture and tradition. This can cause difficulty in debating finer points of doctrine with theologians of different confessions if they don’t or can’t agree on a formal principle.

Lutheran theologians tend to balk at the idea of having a material principle. Some will like to point out that Scripture serves as both the formal and material principles in Lutheran theology. I like the idea, but I also acknowledge that doing so runs the risk of having too vague a material principle. To that end, you may have heard the statement that Article IV is the article on which the church stands or falls; this is referring to Article IV of the Augsburg Confession: “Also [our churches] teach that men cannot be justified before God by their own strength, merits, or works, but are freely justified for Christ’s sake, through faith, when they believe that they are received into favor, and that their sins are forgiven for Christ’s sake, who, by His death, has made satisfaction for our sins. This faith God imputes for righteousness in His sight.”

I’ll begin with a remark by Martin Luther, condemned by the church for proclaiming that people are saved by grace through faith alone on account of Christ. Luther once expressed that, even when he was thoroughly intimidated by his enemies, he could not stop preaching the Gospel: He would envision the Jesus on Judgment Day asking him, “Why were you more afraid of them than me?” He may well have had today’s Gospel in mind.

Jesus was sending His disciples out into the world to preach a specific message: “Repent! The kingdom of heaven is at hand.” As they did so, they would work miracles and wonders. That is the good part, but Jesus also pulled no punches: they would also face heavy opposition.

They would be rejected—sometimes by households, sometimes by whole towns…sometimes by family members, sometimes by close friends, and sometimes by the heads of state and church. “Behold, I am sending you out as sheep in the midst of wolves,” He had just told them (as you may recall from last week). Then He explained what He meant in today’s text, a continuation of last week: “A disciple is not above his teacher, nor a servant above his master…If they have called the master of the house Beelzebul, how much more will they malign those of his household.”

The Pharisee’s asked Jesus’ disciples, “Why does your teacher eat with tax collectors and sinners?” If you really think about it, it shows that they didn’t know Jesus, didn’t know about themselves or those with whom He ate. Who is Jesus? He’s the Son of the Living God. He’s the Christ, the Messiah. He is the sinners’ Savior. That is what His name means: Jesus, Y’shua, “YHWH saves,” that is, “I Am Salvation.” There was no one among them or those with whom Jesus ate who is not a sinner. The same is true of everyone here. As St. Paul wrote, “The saying is trustworthy and deserving of full acceptance, that Christ Jesus came into the world to save sinners, of whom I am the foremost.” (1 Timothy 1:15)

Why did Jesus eat with sinners? Non-sinners don't need Him…they don’t even recognize Him. Furthermore, if He did not eat with sinners, He would have to eat alone.

On this Holy Trinity Sunday, you are confronted once again with this sobering reality: the Church, and you a part of it, worship one God in Three Persons—the Father, uncreated, incomprehensible, eternal; the Son, uncreated, incomprehensible, eternal; the Holy Spirit, uncreated, incomprehensible, eternal. Yet, they are not three uncreateds, incomprehensibles, and eternals, but one. This is not the kind of god that man understands (He is, after all, incomprehensible); this is not the kind of god that man creates!

Nevertheless, this is the way God revealed Himself. God says to you in His Word, “I AM, and this is how I am.” He doesn’t add the caveat, “And you must fully understand it, too.” This side of eternity, no one will fully understand it. One may try to explain it using analogies, such as triangles and shamrocks and the Kool-aid Man, but even these do not allow for full comprehension and may even border on heresy, or fully cross over that border. No, He simply says, “I AM, believe it,” then gives the faith to believe it, so you, by faith, say, “Amen. Yes, yes, it is so.”

He says, “I AM.” God reveals Himself as Trinity. Certainly, you can search the Scriptures front to back and not find that term. You won’t find God outright declaring that which was confessed this morning in the Athanasian Creed as concisely and succinctly as that, but He does reveal His triune-ness. In the beginning, God spoke and things were; “Let us make…,” God said, and it was. (cf. Genesis 1:1—2:4) And when Jesus was walking the earth with His disciples, He had said, “I and My Father are one.” (John 10:30) And, by holy inspiration, the apostle John wrote in one of his letters, “[T]here are three that bear witness in heaven: the Father, the Word, and the Holy Spirit; and these three are one,” (1 John 5:7 NKJV) having once already referred to the Son as the Word. (cf. John 1:1-14)

The catholic faith is this, that we worship one God in Trinity and Trinity in Unity, neither confounding the Persons nor dividing the Substance. For there is one Person of the Father, another of the Son, and another of the Holy Ghost. But the Godhead of the Father, of the Son, and of the Holy Ghost is all one: the glory equal, the majesty coeternal.

I know I jumped a week ahead as you’ll confess this and the rest of the Athanasian Creed for Holy Trinity Sunday, saying in no uncertain terms that you believe in one God in three Persons, distinct yet the same God. I bring up this portion of the Athanasian Creed because today is a day set aside to acknowledge the work of one of the Persons in particular—the Holy Ghost—and when speaking of the work of the Holy Ghost, one cannot escape speaking of the work of God in three Persons; one, because He is one God always at work in all three Persons whether it is recognized or acknowledged or not; and two, because the work of the Holy Ghost is almost always behind the scenes, if you will, calling attention primarily to the Son and His work.

But on the day of Pentecost, particular attention is paid to the Holy Ghost, and His work is more overt and pointed out in the Scripture. Yet, even on this day, the purpose for which the Holy Ghost acted openly was to point to the One who was and is God-with-us and His work for your salvation.