The believers had just heard that the rulers had threatened them and forbidden them to speak in Jesus' name (Acts 4:18, 21-23). In prayer they reach for Psalm 2, where God had already asked why the nations rage against Him and His anointed. They do not ask the question because they are confused. They ask it because God had foretold this very opposition, and they see it being fulfilled. Barnes thinks "the more probable opinion" is that the psalm "referred directly and solely to the Messiah." The question frames their trouble: the rage of rulers against Christ was foreseen, and it is futile.

The prayer itself answers the question. Herod, Pontius Pilate, the Gentiles, and the people of Israel had gathered against "thy holy child Jesus" (Acts 4:27), but only "to do whatsoever thy hand and thy counsel determined before to be done" (Acts 4:28). Their rage could not go beyond what God's counsel had determined. The believers then asked, "Lord, behold their threatenings," and prayed for boldness (Acts 4:29-30). God answered: "the place was shaken where they were assembled together; and they were all filled with the Holy Ghost, and they spake the word of God with boldness" (Acts 4:31).

The question quotes Psalm 2:1-2. Barnes notes that the passage "is an exact quotation from the Septuagint," the Greek translation of the Old Testament. Gill writes that it was "the sense of the ancient doctors of the synagogue, that this psalm is to be understood" of the Messiah, and that the Jews of the apostles' time commonly counted it as David's psalm.

The Greek word for "rage" is phruasso, which Thayer defines as "to neigh, stamp the ground, prance, snort; to be high-spirited." Strong's concordance gives "to make a tumult." It appears only here in the New Testament. Barnes says it refers to "the excitement and tumult of a multitude," not a settled plan. "Imagine vain things" describes plans that come to nothing.

This question shows that God is not surprised or threatened by opposition to His Son. Psalm 2 asks why the nations rage, and the believers apply it to the cross: rulers and peoples gathered against Jesus, yet they only did what God's hand had determined (Acts 4:27-28). Even the worst act in history served God's purpose.

That truth changed how the church prayed. They opened by calling God the Maker of heaven and earth (Acts 4:24), and they did not ask for their enemies to be stopped. They asked for boldness to keep speaking (Acts 4:29). Matthew Henry (Concise) puts it this way: "In threatening times, our care should not be so much that troubles may be prevented, as that we may go on with cheerfulness and courage in our work and duty."