Acts 4:1-4. As Peter is speaking, the priests, the captain of the temple, and the Sadducees come upon the group. These men are annoyed that the apostles are teaching the people and proclaiming the resurrection of the dead through Jesus Christ. So they have the apostles who are there arrested and jailed until morning. But many who had heard them believed, "and the number of the men came to about five thousand."

The priests and Sadducees are, for now, only annoyed at the apostles and they jail them for only one night, hoping, I assume, that that will be the end of it. They have moved on from the crucifixion of Jesus; they think that they have gotten rid of the real problem and now His bothersome apostles will eventually give up. They don't know, or won't let themselves know, about the Resurrection. (The Sadducees didn't believe in the resurrection of the dead at all, much less the Resurrection of this one man.)

The truth, though, will not be suppressed. Already the apostles have garnered thousands of believers. The Holy Spirit is at work and will continue to be. And He didn't send the apostles right to the priests to proclaim Jesus, the priests would have cut them off before they started. Instead, they proclaim to the people and gain thousands so that by the time the priests become aware that they have a real problem on their hands, it will be too late - there will already be too many followers to quash (not that they and the Romans won't try!)