The Holy Spirit was promised by Jesus in John 14. In John 14:26 Jesus said the Holy Spirit would teach believers all things and bring to remembrance those things which Jesus had taught the disciples.
In the book of Acts the Holy Spirit was given in the upper room on the day of Pentecost. Acts 2:1-4 explains how the Holy Spirit dramatically came upon that group of believers. This happened so the believers in Jesus as the Messiah would realize that a new age had started: the Church Age. The coming of the Spirit in such a dramatic fashion caused a break from Old Testament Judaism. Now the Holy Spirit would come and permanently indwell believers when they accepted Christ as their Savior.
The Spirit also served as guarantee or seal that we would eventually inherit eternal life. In Ephesians 1:13 the believers are told that they were sealed with the Holy Spirit the moment they believed. This sealing was like the seal on an old scroll that guaranteed its authenticity. You know by the indwelling Spirit that God is present in your life. Ephesians 4:30 repeats the idea that we are sealed by the Holy Spirit until the day we are redeemed or taken to Heaven.
In Acts l9:1-6 Paul met a group of men who were disciples of John the Baptist. Paul asked them if they had received the Holy Spirit. When they answered that they did not even know that the Holy Spirit existed Paul assumed they were not Christians. Paul explained who Jesus was and baptized them after they believe and then they were filled with the Holy Spirit.
It is impossible therefore to be a Christian and not have the Holy Spirit. This is emphatically declared in Romans 8:9. The Spirit guides us into all truth so we are able to interpret scripture. In John 16:8 it says that the Spirit convicts the world of sin, righteousness, and judgment. We can take comfort in the fact that Jesus sent us a Comforter to guide our lives and convict us when we do wrong. We can rejoice that God is living in us when we become a Christian.