The Book of Acts (in the New Testament of the Bible) is also called 'Acts of the Apostles'. The author of Acts was Luke, who also wrote the Gospel of Luke.
Acts tells the story of the first Christians. After Jesus went up to heaven, they waited in Jerusalem. Then, God sent his Holy Spirit (Acts 2). The Holy Spirit gave power to the first Christians. So, they told the people about Jesus. The Christians explained that the people must confess their evil deeds to God. Then, they must invite God into their lives.
The first Christians did wonderful things by God's power. Peter cured a man who could not walk by God's power (Acts 3:1-10). God even cured ill people when Peter's shadow went over them (Acts 5:15-16).
Many people opposed the first Christians. A Christian called Stephen did many wonderful things (Acts 6:8). But the rulers opposed him. They did not want him to speak about Jesus. So, the rulers killed Stephen. But Stephen was not afraid. He knew that he was going to heaven. Like Jesus, Stephen prayed for the people who were killing him (Acts 7:60).
But the number of Christians continued to increase. And new churches began in many areas, for example Samaria and Antioch. Soon, God taught Peter and the other Christians that God's good news is for everyone, from every nation (Acts 10). So, they began to speak to people who were not from Israel.
Saul was a religious leader who opposed the first Christians. Saul even helped to kill Stephen (Acts 8:1). But God spoke to Saul on the road to Damascus. And Saul became a Christian (Acts 9). He travelled to many places, to tell everyone about Jesus (Acts chapters 13-28). Later, Saul changed his name to Paul. Paul wrote many books in the New Testament of the Bible.
The first Christians had many dangers and many troubles. But they trusted God. And they told people about Jesus in each place where they went. God was with them. God did wonderful things wherever they went. And boldly, they taught the message of the Bible.