What is the indwelling of the Holy Spirit?

At the moment of salvation, every believer is permanently indwelt by God the Father, God the Son, and God the Holy Spirit (Rom. 8:9–11). Indwelling is a once-for-all event that cannot be changed or lost; the Spirit indwells every believer at all times (Rom. 8:9; 1 Cor. 6:19–20; Gal. 3:2; 4:6). This is distinct from the filling of the Spirit: indwelling supplies an inherent power that filling activates (Eph. 5:18), and while indwelling is permanent, filling is a moment-by-moment choice that can be lost through sin and regained through confession (1 John 1:9). In the Old Testament the Spirit came upon certain individuals for specific purposes and could be taken from them; in the Church Age, beginning at Pentecost (Acts 2), He permanently takes up residence in every believer.