“And I will ask the Father, and he will give you another Helper, to be with you forever, even the Spirit of truth, whom the world cannot receive, because it neither sees him nor knows him. You know him, for he dwells with you and will be in you” (John 14:16-17).
John 14-17 has been traditionally called the “farewell discourse.” Jesus is preparing his disciples for his coming departure. As to be expected, the disciples are anxious and concerned about their master leaving. Jesus aims to comfort and he does so by explicating some of the richest Trinitarian theology in all Scripture.
Jesus is leaving but the disciples will not be alone. He assures them that his effectual prayer will open heaven and God’s Helper will come down. The Spirit of truth will descend, an answered prayer, a divine gift.
The coming Spirit signals God’s enduring support and presence. By the Spirit God will now dwell eternally with his people (τὸν αἰῶνα). This text removes any possibility of divine withdrawal, for eternity.
John goes a step further. The eternal presence of God with us shifts to God in us. This is an unprecedented move. D.A. Carson captures this in his book on the farewell discourse.
“One of the most remarkable aspects of Jesus’ teaching in this passage, however, is that it is the triune God who takes up his dwelling in the disciples of Jesus. This truth is unavoidable: ‘I will ask the Father and he will give you another Counselor to be with you forever—the Spirit of Truth.’ . . . The Old Testament writers were concerned that God should live with men [citing 1 Kgs 8:27; Ezek 37:27; Zech 2:10] . . . John insists that this occurred historically in the incarnation: ‘The Word became flesh and lived for a while among us’ (1:14). But now we are brought a stage further: this God reveals himself to the individual believer and takes up residence within him [citing 2 Cor 6:16; Lev 26:12; Jer 32:38; Ezek 37:27; Eph 3:16, 17a; Rev 3:14-21].”
The indwelling Spirit is an unparalleled gift. Jesus tells the disciples a little later that it exceeds even his incarnate presence with them (Jn 16:7). Who can fathom it? God residing in man!
Generosity has always characterized the one true God. He is ever giving the greatest gift to his creatures, himself. This is but another stage in God’s self-donation to broken, believing humanity.
The text indicates the particular role of the Spirit as he indwells believers. He will be another helper. Like the Son of God he will serve and support. The text does not indicate specifics of his helping role. The rest of the New Testament helps us flesh this out.
A few verses later Jesus elaborates on the dynamic of indwelling. “If anyone loves me, he will keep my word, and my Father will love him, and we will come to him and make our home with him” (Jn 14:23).
The previous verses indicate the Spirit’s residence within us. This verse expands the concept of indwelling to include the Father and Son. We are talking about the Trinity living within. God is indeed with us and now in us!
The implications of this are staggering. God has graciously welcomed us into his fellowship. More than that, he has stooped low and come to us. He has come knocking on our doors and has made himself a home within us.
Coming full circle we must remember that Jesus is speaking comfort to his disciples. The promise of indwelling is intended to bring confidence, peace, and hope. The doctrine here is pastoral to the core.
 D. A. Carson, The Farewell Discourse (Grand Rapids: Baker, 1980), 46-47.