They saw what seemed to be tongues of fire that separated and came to rest on each of them. All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them.(Acts 2:3-4)
They were all filled with the Holy Spirit. The unity of the early church came about because they were all right with God and filled with the Spirit of God.
This unity produced amazing results: They preached the same message (2:11); they believed the same things (2:41-44a); they carried the same burdens (2:44b-45; 4:34-35); they loved the same things (2:46-47).
One of the problems in the modern church is that the members are in all different stages of spiritual development. Some are saved but immature. Some are saved and growing. Some are saved and Spirit-filled. Some are not even saved. This diversity makes for difficulty in the family.
There is no way around diversity. But God’s will for us every single saint is to be filled with the Spirit (Eph. 5:18). We are to be controlled by the Holy Spirit. When the Holy Spirit control us, we will walk together in His power for His glory!