Rom 8:9 But you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. Now if anyone does not have the Spirit of Christ, he is not His.
To mind the things if the flesh is indeed death, it is under the dominion of the fleshly impulses of the body. It all works against the laws of God but those He has saved are under the new influence of the Spirit.
The Spirit spoken here is both of God and Christ. The mark of a true believer is the evidence of the Holy Spirit’s indwelling and His continuous work (inside job) evidenced by fruit a Christian displays (outworking of the internal). We live out what He does within. He who is under the sway of fleshly passions is at war with God and cannot please Him and is none of His. But the Christian is under the influence of the Spirit if the Spirit of God dwells in him.
The proof that we are His and have this indwelling Spirit is that we “mind the things of the Spirit and bear its fruit”. Our spirit is made alive by union with Christ, and partaking of His righteousness. We are grafted as branches in Christ the true Vine. Our life sustenance is from our life source Jesus Christ through the Holy Spirit.