Jesus promised his disciples and all who would receive him the gift of the holy spirit. God through Christ reconciled the world to him and actually dwells within the saints who are now the temple of God. The purpose of the holy spirit is to glorify God and Christ. Jesus said he will testify of me. When we are yielded to and filled with the holy spirit he will use you to magnify and exalt Jesus here on Earth.
John 6:63-64 It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life. But there are some of you that believe not. For Jesus knew from the beginning who they were that believed not, and who should betray him.