The Bible tells that there is one God with three Persons: the Father, the Son and the Holy Spirit. Each of these three Persons has his own mission. The Father created the world and sustains it. The Son is born into human flesh and saves the world from its sin. The Holy Spirit awakens and builds faith in a human heart. Therefore we should actually ask: ”Who is the Holy Spirit?”, because the Holy Spirit is the third person of God, not some impersonal power.
The Holy Spirit works among people by awakening and strengthening a persons’ faith in Jesus. He works through the means of grace: the Word of God, Baptism, the Holy Communion and the Confession of sins and Absolution. These are the tools of the Holy Spirit.
Jesus called the Holy Spirit ”the Spirit of Truth” and ”the Defender”. As the Spirit of Truth the Holy Spirit led the writers of the Bible to write according to God’s will, and to us He gives understanding when we read the Bible. As the defender He confirms to our hearts that God is merciful to us through Jesus Christ even though we have fallen and deserved punishment.
In believers the Holy Spirit produces a life that is in accordance with God’s will. He also gives spiritual gifts to upbuild the congregation.
But when the Helper comes, whom I will send to you from the Father, the Spirit of truth, who proceeds from the Father, he will bear witness about me.
(John 15:26, ESV)
The Holy Spirit can live in a human. Actually every believer in Jesus Christ has the Holy Spirit. Without the Holy Spirit one cannot believe. One of the wonders of Christianity is that the almighty God wants to come and live in the heart of a small and sinful person.
The Holy Spirit comes into a person's heart through the Word of God and the sacraments, baptism and Holy Communion. By using these ”means of grace” the Holy Spirit awakens in us a saving faith in Jesus Christ and makes one a child of God. However, a child of God may, through disobedience, grieve the Holy Spirit and fall back into unbelief. That is why the Bible warns about hardening of the heart and about apostasy, and advices one to return to the only Savior.
You, however, are not in the flesh but in the Spirit, if in fact the Spirit of God dwells in you. Anyone who does not have the Spirit of Christ does not belong to him.
(Romans 8:9, ESV)
When scholars claimed that Jesus was working through the power of Satan Jesus warned them of the blaspheming of the Holy Spirit. The blaspheming of the Holy Spirit is conscious opposition of the work of the Holy Spirit. There are similar exhortations elsewhere in the Bible too. The letter to the Hebrews speaks of conscious sin, which leads to a permanent separation from God. Sin in this context does not mean just any sin, but a deliberate denial of faith.
The words of Jesus and the letter to the Hebrews is not to be understood as a condemnation to anyone, but as a warning to those who have not yet committed this irreversible sin. It is important to remember that the door on God’s side is open as long as the good news of Jesus Christ are being proclaimed. The problem for one that is guilty of blaspheming the Holy Spirit is not whether God would forgive him or not, but his unwillingness to return to God - the heart has been hardened. Therefore he who is afraid of having committed the irreversible sin, can be sure that he has not done it yet, because repentance is a sign of a heart that still listens to God’s calling.
And whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.
(Matthew 12:32, ESV)