Ephesians 4:4 says that there is one Body and one Spirit. The one Spirit is the reality of the one Body.
The one Spirit is the Spirit of God in God's creation (Gen. 1:2).
In God's relationship with man, the Spirit is the Spirit of Jehovah (Num. 11:29; Judg. 3:10).
The Spirit is also the Spirit of holiness in the life of God's people in the Old Testament (Psa. 51:11; Isa. 63:10-11). The Spirit in the Old Testament was not the Holy Spirit but the Spirit of holiness. This title is used only three times in the Old Testament, as referred to above.
The title Holy Spirit is used in the New Testament from Christ's incarnation and onward (Luke 1:35; Matt. 1:18). God is holy and His Spirit is holy to make us as holy as God is. By this Holy Spirit, we are joined to God as one (1 Cor. 6:17).
The one Spirit is also the Spirit for the believers of Christ in God's New Testament economy. The Spirit was not yet before Jesus was glorified (John 7:39). The Spirit of God was there from the beginning (Gen. 1:1-2), but before the Lord Jesus was glorified in His resurrection (Luke 24:26), the Spirit as the Spirit of Christ (Rom. 8:9), the Spirit of Jesus Christ (Phil. 1:19), was not yet. Jesus was glorified in His resurrection, and through His resurrection He as the last Adam became a life-giving Spirit (1 Cor. 15:45b), that is, the Spirit. Today the Spirit is the all-inclusive Spirit of Jesus Christ as the living water for us to receive (John 7:38-39).
The one Spirit is now the all-inclusive Spirit, the compound Spirit (cf. Exo. 30:23-28), compounded with Christ's divinity, Christ's humanity, the effectiveness of Christ's death (Rom. 8:13b), and the power of Christ's resurrection (Phil. 3:10). The compound Spirit is also the anointing ointment to anoint us (2 Cor. 1:21-22; 1 John 2:20, 27) and the indwelling Spirit (Rom. 8:11). In the Old Testament, the Spirit of God never dwelt within anyone, but today He indwells every believer of Christ.
This all-inclusive Spirit is the life-giving Spirit (1 Cor. 15:45b; 2 Cor. 3:6b), the Spirit of life (Rom. 8:2), the Spirit of power (Luke 24:49; Acts 1:8), the Spirit of Jesus in His humanity with His suffering (Acts 16:7), the Spirit of Christ in His divinity with His resurrection (Rom. 8:9b), the Spirit of Jesus Christ in both His suffering and resurrection (Phil. 1:19b), and the Spirit of the Lord (2 Cor. 3:17-18) as the Lord Spirit in the transformation of the believers of Christ. Furthermore, the all-inclusive Spirit is the consummated Spirit—the Third of the consummated Triune God (Rev. 22:17; Matt. 28:19)—and the consummation of the processed and consummated Triune God (John 7:39; Rev. 22:17). Eventually, the all-inclusive Spirit is the totality of the processed and consummated Triune God. We need to study all of these points. We must learn to know the Spirit.