Conversion, or “the new birth” (John 3:3), is a miracle of grace worked in the heart by the Holy Spirit. He is the Spirit of God, so that we can say we are “sons of God” (Galatians 4:6) and “partakers of the Divine nature” (2 Peter 1:4). He is the Spirit of Christ, so that Paul could say the mystery of the gospel is “Christ in you …” (Col. 1:27).
This indwelling Spirit of God is integral to the gospel promise (Acts 1:4). Without Him there is no salvation. If we are sons of God we have the Spirit and if we have the Spirt we are sons of God. It is by the Holy Spirit that we experience the joy of Christ (John 15:11) and the peace of God (Philippians 4:7-9). He is the power for the outworking of our salvation (Philippians 2:12). Therefore, we must be “filled with the Spirit …” (Ephesians 5:18).