(Acts 1 and 2)
After Jesus rose from the dead, He told the apostles to wait in Jerusalem until they received power when the Holy Spirit came on them. Forty days after Jesus rose, he went back to be with God the Father. As He talked with His disciples, He started rising up into the air and finally went into the clouds. The disciples were very happy and so they waited in Jerusalem as Jesus had said, praying all the time. They chose Matthias, who had been with them since Jesus' baptism, to take the place of Judas Iscariot, who had killed himself.
Ten days later, when the apostles were all gather together in one room, the Holy Spirit came on them like Jesus had said. It was on the Jewish feast day of Pentecost. They heard a noise from the sky like a strong wind blowing. Then they saw something that looked like flames of fire that separated and stood over each of them. They were all filled with the Holy Spirit and began to speak in different languages, as the Holy Spirit gave them the power to speak.
Now Jews from all over the world were in the city of Jerusalem at that time. When people heard the apostles, a large crowd came together. Every person heard the apostles praising God in their own language. Now they were surprised that each person could hear in the language he was born in. So they started asking, "What is happening?" But other people were making fun of the apostles and saying they were drunk.
Then Peter stood up with the other eleven apostles. He told the crowd that they were not drunk, but that what the prophet Joel had said would happen was happening. He told the Jews there that God had sent Jesus to them, but that they had nailed Him to a cross. But God had known that this would happen so He raised Jesus from the dead. This is what King David had been talking about in the Psalms when he wrote the song to God that says:
"You will not leave My soul in Hades [that is, the world of the dead];
You will not let Your Holy One's body rot."
Then Peter said that David was still dead, but that Jesus was alive. Now He had gone up into heaven to sit at the right side of God. The Father had given Jesus the Holy Spirit and He had poured that Spirit on His followers.
When the people heard this, they were very, very sorry. They asked, "What should we do?" Then Peter told them, "Repent [that is, change your mind about sinning], and each of you must be baptized in the name of Jesus Christ. Then God will forgive your sins and you will receive the gift of the Holy Spirit." Peter preached to them some more. Then those who believed what he said were baptized. That day 3,000 people became Christians. It was the beginning of Christ's church. The Christians listened to what the apostles taught, shared what they had, ate the Lord's Supper together, and prayed together. And every day more and more people became Christians.
That church continues down to this day. It is not an organization that you join. It is everyone who is truly a Christian. When a person believes in Jesus, repents of his sins, and is baptized into Jesus, then God saves him. And God adds everyone who He saves to His church. You too can be saved by Jesus and belong to His church. You do not have to join a denomination; in fact, since God never commanded you to, you should not. But you should meet with a local group of Christians to study the apostle's teaching, share together, eat the Lord's Supper together, and pray together. Jesus came and died to save you. But whether He will or not is up to you. What will your answer be?