Why do people accept Pentecost as the beginning of the Church? Is there any direct Biblical teaching on this?

When Jesus left his disciples to go to Heaven, he promised them that they wouldn't be left on their own, but that the Holy Spirit would be with them. when they were gathered together to pray, the Holy Spirit came down on them. This was a new phase for them. Jesus was gone in the body, and they were to carry out his teachings, but the Holy Spirit would empower them to do what they had to do. These people were spreading the gospel of Jesus, which made them different from the Old Testament chosen people. They lived in faith of the coming Messiah. These people had known Jesus and knew his message. Now they could be the "church" (or ecclesia, called-out).