The Scriptures teach us certain doctrines by explicit development in specific passages—justification by faith (Rom. 3 and 4, Gal. 2 and 3), salvation by grace alone (Eph. 2:1–10), the power of the preaching of the gospel (1 Cor. 1). Other doctrines we learn only by gathering up Scriptural teaching in many places and putting it all together. The doctrine of the Trinity belongs to the latter category. The Scriptures do not give us anywhere a formulation of that doctrine similar to what we find in the Nicene or Athanasian Creeds. Nevertheless, we can come to a proper understanding of it by comparing Scripture with Scripture.
The same is true of the writings of the Apostle Paul. We do not find anywhere in his letters a precise formulation of this fundamental doctrine of our faith. Yet he not only assumes the doctrine but even makes it foundational to his theology. This is perhaps nowhere more true than in the letter to the Ephesians. Especially in the theological part (chapters 1–3), but also in the hortatory part (chapters 4–6), Paul many times makes plain that all of his instruction and exhortation depends on the reality of the Trinity. Understanding the doctrine of the Trinity is essential to understanding his theology.
The first place we find the doctrine of the Trinity assumed in the letter is in the doxology (1:3-14). That doxology is one long sentence, though our modern translations usually break it up into three. The sentence has three parts, and each of these parts pertains particularly to one person of the Trinity. In the first part (vv. 3–6) Paul explains the blessings we receive from the Father: Blessed be the God and Father of our Lord Jesus Christ who has blessed us with every spiritual blessing… In the second part (vv. 7–12) he recounts the blessings we have received from the Son in whom we have redemption through his blood, the forgiveness of sins… And in the third part (vv. 13–14) he praises the Holy Spirit. You were sealed with the Holy Spirit of Promise, who is the guarantee of our inheritance.
In the last part of the fist chapter (vv. 15–23) the apostle describes his prayer for the Ephesian saints, and this prayer is Trinitarian. He prayed that the God of our Lord Jesus Christ, the Father of glory, may give to you the spirit of wisdom and revelation in the knowledge of Him. Our translation has put a small “s” on the word spirit, as if it referred to the human spirit. However, the word should be capitalized. Paul alludes to Isaiah 11:2: The Spirit of the LORD shall rest upon Him [the rod from the stem of Jesse], The Spirit of wisdom and understanding, the Spirit of counsel and might, the Spirit of knowledge and of the fear of the LORD. Paul, therefore, prays to the God and Father of our savior, the Lord Jesus Christ, that he will give to us the Holy Spirit who enlightens and makes wise.
The famous passage on salvation by grace alone (2:1–10) refers only to the Father and the Son, but the passage immediately following (2:11–18) mentions all three. Verse 18 teaches us that through Him [Christ] we both [Jew and Gentile believers] have access by one Spirit to the Father.
The next paragraph, which compares the church to a temple, says, in the Lord [Jesus Christ]… you also are being built together for a dwelling place of God in the Spirit.
Chapter 3:1–7 describes the revelation of the mystery of the gospel to Paul and the other apostles and prophets. This revelation was the gift of the grace of God (v. 7) to him, it is the mystery of Christ (v. 4), and the Spirit gave it (v. 5).
Near the end of chapter 3 (vv. 14–19), the apostle again explains to the Ephesians how he is praying for them. This prayer forms a kind of rough chiasm based on the doctrine of the Trinity. Paul begins by bowing his knees the Father of our Lord Jesus Christ that He would grant you to be strengthened with might through His Spirit in the inner man, and he closes with the request that they may know the love of Christ which passes knowledge; that you may be filled with all the fulness of God.
The most important exhortation in the epistle, the exhortation to keep the unity of the Spirit in the bond of peace (4:1–7) is Trinitarian, for there is one body and one Spirit, just as were called in one hope of your calling; one Lord, one faith, one baptism; one God and Father of all, who is above all, and through all, and in you all. One Spirit, one Lord, one Father—one Triune God—the creator of the one new man in Christ, the one church composed of Jews and Gentiles.
Other exhortations in the rest of the letter do the same thing. 4:25–32 exhorts us not to grieve the Holy Spirit of God and to be kind to one another, tender hearted, forgiving one another, even as God in Christ forgave you. The exhortation to be imitators of God refers to God and Christ (vv. 2 and 5), and grounds the exhortation to walk as children of light in the fruit of the Spirit (v. 9). Verses 17–21 of the same chapter exhort us to be filled with Spirit and give thanks always for all things to God the Father in the name of our Lord Jesus Christ. Each person appears multiple times and each time in his specific role in the works of God, the Father as the cause, origin and beginning of all things, the Son as the word, wisdom and image of the Father, and the Spirit as the eternal power and might (cf. Belgic Confession Article 8). The Father predestines and adopts, the Son redeems, and the Spirit indwells. The book of Ephesians all by itself contains a pretty complete doctrine of the Trinity, and overlooking it truncates and even distorts our understanding of the glorious gospel and the mystery of our salvation.