At that day ye shall know that I am in my Father,
and ye in me, and I in you.
John 14:20
It would have to do with the Spirit of God that dwells with God being in Christ and in us.
2 Corinthians 5:19 To wit, that God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them; and hath committed unto us the word of reconciliation.
Colossians 1:26-27 Even the mystery which hath been hid from ages and from generations, but now is made manifest to his saints: 27To whom God would make known what is the riches of the glory of this mystery among the Gentiles; which is Christ in you, the hope of glory:
It is the promise of God.
Ezekiel 36:26-27 A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh. 27And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do them.
The OT saints could not receive the promise of the indwelling of the Spirit as Christ had not made the atonement.
Hebrews 11:39-40 And these all, having obtained a good report through faith, received not the promise: 40God having provided some better thing for us, that they without us should not be made perfect.
John 7:39 (But this spake he of the Spirit, which they that believe on him should receive: for the Holy Ghost was not yet given; because that Jesus was not yet glorified.)
The gift of God is salvation but salvation entails Christ’s atonement, payment of sins, our believing in him, forgiveness of sins, and the indwelling of the Spirit.
Romans 8:9-11 But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his. 10And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness. 11But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you.
God, Christ, the Spirit, dwells in those that have faith toward God, in the Lord Jesus Christ.
1 Corinthians 3:16 Know ye not that ye are the temple of God, and that the Spirit of God dwelleth in you?
I hope this aids you in understanding the Scriptures.