As already discussed the Holy Spirit is the third part of the trinity. In the Old Testament the Holy Sprit would come upon certain individuals at certain times in and gave them supernatural power in order to do exploits for God. For example it says of Samson “The Spirit of the LORD came upon him in power”
However, the prophets looked ahead to the day when the Holy Spirit would permanently indwell all who believed: “I will pour out my Spirit on all people” (Joel 2v28). When Jesus was coming to the end of His ministry this is what He spoke to His disciples about. He told them He wouldn’t leave them as orphans but would send someone like Him (the Holy Spirit), who would live on the inside of them (John 14-16).
The disciples were to continue the ministry that Jesus had started (Acts 1v1). They were to go into all the world and preach the gospel (Matt 28). Supernatural signs were to follow their ministry (Mark 16). Indeed they were to accomplish even greater things than Jesus did (John 14v12). It was clear that the disciples couldn’t do any of these things in their own strength. That is why Jesus told them “Do not leave Jerusalem, but wait for the gift my Father promised, which you have heard me speak about. For John baptised with water, but in a few days you will be baptised with the Holy Spirit.” (Acts 1v4-5). The word baptize means “to immerse”. Jesus was saying the disciples would be filled or immersed with the Holy Spirit. This would give them supernatural power to live the Christian life and fulfill the commission of Jesus: “you will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth.” (Acts 1v7-8)
In Acts 2 you can read how the disciples were baptized in the Holy Spirit. This baptism is essential for us today. We too need the power of God in our lives so we can serve Jesus effectively. Peter said that the baptism for the Holy Spirit was “for you and your children and for all who are far off— for all whom the Lord our God will call.” (Acts 2v39).
The Holy Spirit is described as the gift of God (Acts 11v17). Therefore we can not earn Him, and we don’t need to do anything to get the gift. We simply need to receive the Holy Spirit by faith. The only qualification is that we have repented of our sin and put our faith in Jesus (Acts 2v38-39)
In Acts 2 as the disciples were filled with the Holy Spirit they “began to speak in other tongues as the Spirit enabled them” (Acts 2v4). In fact on each occasion when someone in Acts was baptized in the Holy Spirit it records them speaking in other tongues. The exception to this in Acts 8, however as Simon “saw” something happening, it is most likely to have been people speaking in tongues. Therefore, we believe that today, as the initial evidence that someone has been baptized in the Holy Spirit they will speak in other tongues, given to them by God.