King Herod arrested Peter because he wanted to please the Jews. At the time, the Christian faith was gaining popularity, and Herod wanted to maintain his own power and control over the people. When James, one of the apostles, was put to death, it pleased the Jewish leaders, so Herod thought he could win even more favor by arresting Peter as well. Herod hoped that by eliminating the leaders of this new religion, he could prevent it from spreading further and causing unrest among the people.