As our Lord continues to give good reasons for his disciples to let faith in God soothe their troubled hearts, he turns in the second half of John 14 to the critical element of the promised Holy Spirit. With the sending of the Spirit, the Father and the Son make their home with the believer as the Spirit dwells in the believer. With this teaching about the Holy Spirit, our Lord complements and even completes his earlier teaching on the mutual indwelling of the God. Earlier in John 14, Jesus told his disciples that “I am in the Father and the Father is in me” (v11 ESV). That mutual indwelling of the Father and the Son now explicitly includes the Holy Spirit as Jesus connects the homemaking of the Father and the Son with the promised Holy Spirit.
The tight connection between the relations of the Father, Son, and Spirit as presented by Christ in John 14, beyond supporting us with encouragement in a weary world, should also help us to be discerning when it comes to our engagement with all sorts of views of God. I’d like to raise three views of God that distort or even ignore the Bible’s teaching about the Trinitarian relations.
First, the pouring out of the Holy Spirit is not something separate and apart from the ministry of Christ. Rather, it is its continuation. Therefore, there can be no “Christless mysticism” possible with a balanced view of God along the lines of how Jesus describes the relationship between Father, Son, and Spirit in our text. This is, to an extent, one of the points that Paul argues against the Corinthian church, which had become puffed up with spiritual pride regarding its so-called spiritual gifts. And yet, without love, which necessarily flows from the mutual indwelling of the Triune God, the Corinthians were just a noisy gong. In short, the Spirit is the Spirit of Christ sent by the Father to continue the work of God, so his work cannot be divorced from God’s work and, at its worst, appropriated for our personal gain.
Second, throughout the history of the church, the three-ness of God has been overemphasized to the point that the truth of monotheism has morphed into the error of tritheism. In tritheism, the persons of the Father, Son, and Holy Spirit become absolute or ultimate and therefore separable. Our God becomes our gods, and the oneness of the true and living God is lost. Jesus’ emphasis on the mutual indwelling of the Father, Son, and Spirit is at least in part aimed at mitigating this error. While we acknowledge that the Father, Son, and Spirit are the three persons of the one God, the Son is in the Father, the Father is in the Son, and the Father and Son make their home where the Spirit resides. There is an equally ultimate oneness that balances the three-ness of our God so that we do not fall into the error of tritheism.
Third, the opposite error is to overemphasize oneness to the exclusion of the real three-ness of God. This overemphasis of oneness can turn into the error of modalism. For the sake of simplicity, we could say that modalism teaches that the sending of the Holy Spirit in the church as is a new mode of God’s singular being, as though God was first Father in the Old Testament, then Son in the New Testament, and now Spirit in the church age. But modalism doesn’t make sense of Jesus’ teaching in John 14. When the Spirit is sent to abide with and in believers, the Father and Son make their dwelling there, too. Our Lord has the idea of the united three-ness of God close at hand as he speaks of the promised Holy Spirit. More than that, modalism has a hard time making sense of events like Jesus’ baptism in which the Father, Son, and Spirit are all present at once. Thus, there is no room for modalism in the Christian church.
In the end, there is a practical takeaway to be had with all of this discussion of Trinitarian relations. Robert Letham says the following on the importance of the doctrine of the Trinity: (ST, 152)
The doctrine of the Trinity tells us that God is personal and relational; love is at the heart of who God is. C. S. Lewis remarked that the Trinity is required for God to be love. “The words ‘God is love’ have no meaning unless God contains at least two persons. … If God was a single person, then before the world was made, he was not love.”
Let us, then, give thanks to our triune God, who is love, who has manifested his love in Christ Jesus, and who has applied his love by his Holy Spirit.
 Robert Letham, Systematic Theology (Wheaton: Crossway, 2019), 152.