Acts 20 contains two references to the Trinity.
And now, behold, I am going to Jerusalem, constrained by the Spirit, not knowing what will happen to me there, except that the Holy Spirit testifies to me in every city that imprisonment and afflictions await me. But I do not account my life of any value nor as precious to myself, if only I may finish my course and the ministry that I received from the Lord Jesus, to testify to the gospel of the grace of God. (Act 20:22-24)
This is a somewhat important reference for the book of Acts because it lays the foundation for what is to occur in the following chapters. The references to the Trinity have been dense in Acts to this point, occurring in almost every chapter. Yet a reference to the Trinity does not occur again in the book until the final verses of chapter 28. But far from being absent, this passage demonstrates that everything happening in chapters 21-28 is the work of the Trinity.
In Acts 20 Paul is departing Ephesus for Jerusalem. From personal witness of the Holy Spirit and the testimony of Spirit-filled prophets, he knows that trouble awaits him in the holy city. But Paul perseveres and continues with his journey. The Holy Spirit does not give Paul the revelation of trouble to warn him, but to strengthen him. Paul knows that this is the work that the Lord Jesus has determined for him. To carry the message of the grace of God in the power of the Spirit is the task assigned to him by his Lord, and he must not fail in the face of difficulty. Paul is emboldened to complete his work by his confidence in the work the Trinity.
But before he leaves, he wishes to share this confidence with those remaining in Ephesus.
Pay careful attention to yourselves and to all the flock, in which the Holy Spirit has made you overseers, to car for the church of God, which he obtained with his own blood. I know that after my departure fierce wolves will come in among you, not sparing the flock; and from among your own selves will arise men speaking twisted things, to draw away the disciples after them. Therefore be alert, remembering that for three years I did not cease night or day to admonish everyone with tears. And now I commend you to God and to the word of his grace, which is able to build you up and to give you the inheritance among all those who are sanctified. (Act 20:28-32)
On the Trinitarian side, this passage seems particularly fruitful for proving the deity of Jesus Christ. Everyone agrees that the mention of “his own blood” is referring to Jesus. The English translations make it seem like Paul is saying, “God purchased the church with his own blood.” In which case, this would be a clear statement that Jesus is God–since Jesus is the only member of the Trinity with blood. Yet commentators are in general agreement that the phrase is more accurately translated, “Which God purchased with the blood his own (Son).” While the word “Son” is not in the text, it is supplied as being the understood meaning (cf. Rom. 8:3, 32). So even if this is not a clear statement of the deity of Christ, the passage is certainly Trinitarian.
On the practical side we see that this strength in the face of opposition is not for a select few, it is for all the church. Satan is not content to merely have God’s children sent in among the lions, he also send wolves into the flock. But the same power of the Trinity that keeps Paul as he travels into enemy territory strengthens all Christians to withstand the foes that seek to sneak among them. The grace of God the emboldened Paul in verse 24, strengthens and protects the church of Jesus Christ in which the Holy Spirit raises up overseers.
On the theological side we also see instruction about the work of the Trinity toward the church. The blood of the Son purchased the church; the Holy Spirit raises up leaders for its protection; the Father strengthens and builds it by his word. Great is our boldness on account of the Father, Son, and Holy Spirit.