John teaches that believers will be in the Father and the Son, Jesus and believers will be in one another, and Jesus and his disciples mutually will abide in one another. In sum: the Father and the Son and believers will mutually indwell one another.
Believers will be in the Father and the Son. Jesus prays for the unity of future believers and uses the unity of the Father and the Son as his measure-"just as you, Father, are in me, and I in you, that they also may be in us" (JN 17:21).
John frequently says that Jesus and believers will be in one another or abide in one another. Jesus predicts, "In that day you will know that I am in my Father and you in me, and I in you" (JN 14:20). Jesus and his true disciples will mutually abide in one another:
"My flesh is food, and my blood is true drink. Whoever feeds on my flesh and drinks my blood abides in me, and I in him. (JN 6:55-56)
"Abide in me, and I in you. As, the branch cannot bear fruit by itself, unless it abides in the vine, neither can you unless you abide in me. I am the vine; you are the branches. Whoever abides in me and I in him, he it is that bears much fruit, for apart from me you can do nothing." John 15:4-5
Because of the Father's love for the Son and their love for the world, and because of the Son's incarnation, death, and resurrection, believers will be in the Father and the Son. Amazing as it sounds, believers will mutually indwell the Trinitarian persons as an act of grace insofar as creatures can partake of the divine life. David Crump's words are pointed: "Mutually indwelling the life of God is the heart and the soul of John's understanding of salvation. . . Every believer's inclusion within the exchange of divine life and the love between the Father and the Son is the essence, the heart and the soul, of his message about eternal life."
John includes the Holy Spirit in the mission of God. Jesus will ask the Father to send the Spirit to indwell and be with believers, and they will know him (JN 14:17). Although John does not correlate the Spirit's work and union with Christ, as does Paul, John provides the raw materials for systematic theology to do so." pg. 70,71 Robert A. Peterson "Salvation Applied By The Spirit: Union with Christ"