The third person of the Trinity.
As noted in 1 John 5:7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. The Holy Spirit was active in creation, Genesis 1:2, the formation of man, Genesis 2:7, and the overshadowed the birth, Luke 1:35, and resurrection of Jesus, Romans 8:11.
Today the Holy Spirit works to testify of Jesus and works to bring unsaved people to the saving knowledge of the gospel of grace. John 16:8-11, describes the work of the Holy Spirit to convict men of sin, shows the need for a Savior.
And when he is come, he (Holy Spirit) will reprove the world of sin, and of righteousness, and of judgment: [9] Of sin, because they believe not on me; [10] Of righteousness, because I go to my Father, and ye see me no more; [11] Of judgment, because the prince of this world is judged.
Once a person understands that they need a Savior, the Holy Spirit renews man.
Titus 3:5 Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost;
The moment a person believes that Jesus died on the cross to pay for their sins, acknowledges that they can do nothing to earn salvation, and accepts the free gift of eternal life, that person becomes a born-again Christian. The Holy Spirit then makes them alive!
John 6:63 It is the spirit that quickeneth (make alive); the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life.
Once a person is a Christian the Holy Spirit’s does many things for us:
Dwells with/in: Romans 8:9, 2 Timothy 1:14
Seals: Ephesians 1:13, 4:30
Comfort: John 16:7
Intercedes/Prays: Romans 8:26-27
Teaches: 1 Corinthians 2:12-14
Empowers: Acts 1:8
We need to remember that as a Christian we are given a free will. We choose how much we allow the Holy Spirit to influence/control our hearts and actions. God warns us that we can grieve the Holy Spirit by not doing God’s will.
Ephesians 4:30 And grieve not the holy Spirit of God, whereby ye are sealed unto the day of redemption.
As Christians, we are commanded to be filled with the Holy Spirit, and we will have fruit!
Ephesians 5:18 And be not drunk with wine, wherein is excess; but be filled with the Spirit;
Galatians 5:22-23 But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, meekness, temperance: against such there is no law.
The secret is to ABIDE IN CHRIST! Without the help of the Holy Spirit, we can do nothing!
John 15:4-5 Abide in me, and I in you. As the branch cannot bear fruit of itself, except it abide in the vine; no more can ye, except ye abide in me. [5] I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing.