Now as they spoke to the people, the priests, the captain of the temple, and the Sadducees came upon them, 2 being greatly disturbed that they taught the people and preached in Jesus the resurrection from the dead. 3 And they laid hands on them, and put them in custody until the next day, for it was already evening. 4 However, many of those who heard the word believed; and the number of the men came to be about five thousand. — Acts 4:1-4.
Peter and John are arrested and placed in custody because the Sadducees, who do not believe in the resurrection, are greatly disturbed by their preaching the resurrection of the Lord Jesus Christ. God blesses their testimony and many of those who heard the word believed. The number of men came to about 5,000.