What does Acts 5:39 mean?

Gamaliel, a highly respected Pharisee, is explaining why the Sanhedrin shouldn't kill Jesus' twelve apostles. In the previous three verses, he points out what happens to religious/political movements that are not in God's will: once the leaders are gone, the movements disappear. He's not entirely accurate, but the most glaring error in his claim won't manifest for another thirty years or more. In AD 70, the remnant left after the defeat of Judas the Galilean (Acts 5:37) will spark a reprisal from Rome that obliterates the temple. The main point he makes, coarse as it is, is that false spiritual movements will eventually fail by the hand of God.

In this verse, Gamaliel gives the positive corollary. It's unclear how he means this and how it is taken. Most men in the room would never admit—yet—that Jesus could be on God's side. But pretending it's a possibility gives them an excuse to save face. They don't have to demand the apostles' execution and risk losing the people's support. Instead, they can posture as if taking the high ground, letting God handle things (Acts 5:13). This is a slightly-more-polite version of their strategy with Jesus and Pilate (Matthew 27:15–23).

Gamaliel's calmness and restraint are not universally shared. His student, Saul of Tarsus, will brutally persecute the Christians (Acts 8:1–3; 22:3). Before long, a mob of Jews from Cyrene, Alexandria, Cilicia, and Asia will debate with Stephen; they'll stone him when they find his conviction that God's worship cannot be confined to the temple blasphemous (Acts 6:8–7:60). We know Saul is at Stephen's murder, guarding the mob's cloaks (Acts 7:58). Since he is from Tarsus in Cilicia, he is possibly involved in the initial debates, as well. Saul goes on to persecute the early believers, chasing them out of Jerusalem (Acts 8:1–3) and beyond (Acts 9:1–2). Fortunately, Stephen is proved right; Jesus meets Saul not in the temple but on the road to Damascus, and Saul responds (Acts 9:1–22).

Proverbs says, "Many are the plans in the mind of a man, but it is the purpose of the LORD that will stand" (Proverbs 19:21). Gamaliel accurately applies this truism to the Sanhedrin's situation. He's only wrong about which side will be defeated by God.