Here is James Houston on the significance that the Holy Spirit plays in the trinitarian life of God:

. . . It is sin, the sin of individualism, which so besets us today, with the breakdown of societal values in conjunction with the building up of technocratic values, to make us incredulous toward and blind to the ‘personalness’ of the Person of the Holy Spirit. And the hazards of professional scholarship and the institutionalism of the churches only intensify individualism. Sanctity needs therefore to be recovered in theological scholarship. And holiness is the test of true personhood, of life poured out sacrificially for the sake of divine love. Without the third Person, we would not experience the love between the Father and the Son, nor would we be recipients of the personal life of God. Inour individualism, we would merely oscillate between a modalistic understanding of God, which knows of no fundamental distinction between the Persons of the Godhead, and a pantheistic understanding, which knows of no ultimate distinction between God and humanity. It is the person of the Holy Spirit that reveals the mystery of the Trinity to us, and by participation in the Spirit enables us to make true confession withi Athanasius: ‘Apart from the Spirit we are strange and distant from God, and by participation of the Spirit we are knit into the Godhead; so that our being in the Father is not ours, but is the Spirit’s which is in us and abides in us, while by the true confession we preserve it in us.’ (Paul Metzger, ed., James Houston, Trinitarian Soundings in Systematic Theology,” 149-50)