The Holy Spirit is the third person of the Trinity — fully God, co-equal with the Father and the Son, not a force, not an it, but a He. He is called the Spirit of God, the Spirit of Christ, the Comforter, the Advocate, the Paraclete, the Spirit of Truth, and the Spirit of Adoption. The Bible begins with Him: "The Spirit of God was hovering over the face of the waters" (Genesis 1:2). He inspired the prophets (2 Peter 1:21), empowered the judges and kings, and descended visibly on Christ at His baptism (Matthew 3:16). Jesus promised Him as "another Comforter" (John 14:16 — another meaning "of the same kind as Me"). At Pentecost the Spirit was poured out on the church (Acts 2), inaugurating the New Covenant era in which every believer is indwelt by God Himself (1 Corinthians 6:19). The Spirit's work includes: convicting the world of sin (John 16:8); regenerating sinners (John 3:5-8, Titus 3:5); indwelling believers (Romans 8:9-11); adopting them and witnessing to their sonship (Romans 8:15-16); sanctifying them (2 Thessalonians 2:13); gifting the church for service (1 Corinthians 12); illuminating the Scriptures (1 Corinthians 2:10-14); praying for them with groanings (Romans 8:26); producing fruit — love, joy, peace, longsuffering, kindness, goodness, faithfulness, gentleness, self-control (Galatians 5:22-23); and raising the bodies of believers (Romans 8:11). To grieve the Spirit (Ephesians 4:30) or quench the Spirit (1 Thessalonians 5:19) is to live in a way unworthy of the One who dwells in us. Christian life is fundamentally Spirit-empowered: "If you live according to the flesh you will die; but if by the Spirit you put to death the deeds of the body, you will live" (Romans 8:13).
John 14:16-17 — "And I will pray the Father, and He will give you another Helper, that He may abide with you forever — the Spirit of truth, whom the world cannot receive."
John 16:7-8 — "Nevertheless I tell you the truth. It is to your advantage that I go away; for if I do not go away, the Helper will not come to you; but if I depart, I will send Him to you. And when He has come, He will convict the world of sin, and of righteousness, and of judgment."
Romans 8:11 — "But if the Spirit of Him who raised Jesus from the dead dwells in you, He who raised Christ from the dead will also give life to your mortal bodies through His Spirit who dwells in you."
Galatians 5:22-23 — "But the fruit of the Spirit is love, joy, peace, longsuffering, kindness, goodness, faithfulness, gentleness, self-control."
Acts 2:4 — "And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit gave them utterance."