God’s complete salvation is in one spiritual, practical, and experiential union of the Spirit of life with our spirit, forming a mingled spirit. The practical union is by the two divine transfers in one spiritual union for us to reign in life by enjoying the abundance of grace and of the gift of righteousness. This union is of the Spirit of life, the Spirit of God, the Spirit of Christ (the pneumatic Christ), and the Spirit of resurrection indwelling our spirit. The one spiritual union is with our spirit, the regenerated spirit of the believers in Christ indwelt by the Spirit, forming a mingled spirit. The two spirits are not only united to be a union; they are also mingled to be a mingled spirit.
The Spirit of life is the Spirit of Christ, and Christ corresponds with the law of God. This Spirit within us spontaneously fulfills all the righteous requirements of the law through us when we walk according to Him. The requirements that we must fulfill in order that the law of the Spirit of life (which has already been installed in us) may work are: (1) to walk according to the spirit (v. 4); (2) to mind the things of the Spirit— to set the mind on the spirit (vv. 5-6); (3) to put to death by the Spirit the practices of the body (v. 13); (4) to be led by the Spirit as sons of God (v. 14); (5) to cry to the Father in the spirit of sonship (v. 15); (6) to witness that we are the children of God (v. 16); and (7) to groan for the full sonship, the redemption of our body (v. 23).
It is difficult to discern the word spirit used in Romans 8, in Galatians 5, and in other places in the New Testament, unless it is clearly designated to denote God’s Holy Spirit or our regenerated human spirit, as in Romans 8:9 and 16. According to the usage in the New Testament, the word spirit as used in Romans 8:4 denotes our regenerated human spirit indwelt by and mingled with the Spirit, who is the consummation of the Triune God (v. 9). This corresponds with 1 Corinthians 6:17, "He who is joined to the Lord [who is the Spirit—2 Cor. 3:17; 1 Cor. 15:45] is one spirit"—one mingled spirit.
Being joined to the Lord refers to the believers’ organic union with the Lord through believing into Him (John 3:15-16). This union is illustrated by that of the branches with the vine (John 15:4-5). It is a matter not only of life but in life (the divine life). Such a union with the resurrected Lord can only be in our spirit. One spirit in 1 Corinthians 6:17 indicates the mingling of the Lord as the Spirit with our spirit. Our spirit has been regenerated by the Spirit of God (John 3:6), who is now in us (1 Cor. 6:19) and is one with our spirit (Rom. 8:16). This is the realization of the Lord, who became the life-giving Spirit through resurrection (1 Cor. 15:45; 2 Cor. 3:17) and who is now with our spirit (2 Tim. 4:22). This mingled spirit is often referred to in Paul’s Epistles, as in Romans 8:4-6.
It is by the all-inclusive compound life-giving Spirit (see note 4 in Phil. 1:19) that we are born of God, we receive the divine life as the divine seed in us, we have the fellowship of the divine life, we are anointed with the Triune God, and we abide in the Lord. This wonderful Spirit is given to us as the promised blessing of the New Testament (Gal. 3:14); He is given without measure by the Christ who is above all, who inherits all, and who is to be increased universally (John 3:31-35, 29a). This Spirit and the eternal life (John 3:15) are the basic elements by which we live the life that abides in the Lord continuously. Hence, it is by this Spirit, who witnesses assuringly with our spirit, that we are the children of God (Rom. 8:16) and that we know that the Lord of all abides in us (1 John 4:13). It is through this Spirit that we are joined to the Lord as one spirit (1 Cor. 6:17), and it is by this Spirit that we enjoy the riches of the Triune God (2 Cor. 13:14).
(Crystallization-Study of the Complete Salvation of God in Romans, Chapter 3, by Witness Lee)