Acts 4:1 & 2

"The priests and the captain of the temple guard and the Sadducees came up to Peter and John while they were speaking to the people. They were greatly disturbed because the apostles were teaching the people and proclaiming in Jesus the resurrection of the dead." NIV translation

The Sadducees that are referred to here were generally the priests of the rich. They did not look for a coming day in fact they did not believe in resurrection, heaven, or hell. They believed that the soul died with the body and basically that was it. Generally speaking, when the New Testament refers to the "chief priests" they are referring to this group of people. With this in mind, it is easy to see why they would be upset about Peter and John speaking to the crowd about Jesus' resurrection. If what Peter said was true, then the Sadducees would be discredited. They would lose all of their power, money, and influence. We may think that this was just back in the history of the early church but there are many people who believe the same type of thing today.

Acts 4:3 & 4

"They seized Peter and John, and because it was evening, they put them in jail until the next day. But many who heard the message believed, and the number of men grew to about five thousand." NIV translation

We may find it hard to believe that you can be thrown in jail just for telling people about Jesus but that is exactly what happened here. The same thing happens in several countries to many Christians even today. You see, this teaching about Jesus endangered the rule of the Sadducees as well as the Pharisees. If the people believed in Jesus then the rulers were done because there was no need for a priest to be between God and man. 

The pattern is still the same today as what happened here. We are told that at the end of the speech to the crowd, the number of men who believed was five thousand. The same thing can happen today, if people boldly tell about Jesus, people will be saved.