Before Jesus was taken into heaven where He was enthroned to share God's rule over the world, He instructed His followers to be a witness for Him. The book of Acts details their witness.
Jesus' followers received the Holy Spirit on Pentecost and became filled and equipped to spread the good news that would restore God's kingdom over the world. Through persecution, the believers were scattered. They continued to preach, and the church at Antioch was born.
It grew into the first multiethnic, international church from which missionaries were sent throughout the world to preach about Jesus, the messianic Messiah and risen king of all nations. Paul even continued to preach despite imprisonment and wrote important letters to the churches.