From The Christian Doctrine of God, One Being Three Persons (Edinburgh: T&T Clark, 1996):
While the Lord Jesus Christ constitutes the pivotal centre of our knowledge of God, God’s distinctive self-revelation as Holy Trinity, One Being, Three Persons, creates the overall framework within which all Christian theology is to be formulated. Understandably, therefore, the doctrine of the Holy Trinity has been called the innermost heart of Christian faith and worship, the central dogma of classical theology, the fundamental grammar of our knowledge of God. It belongs to the Gospel of God’s saving and redeeming love in Jesus Christ who died for us and rose again and has given us the Holy Spirit who has shed the love of God abroad in our hearts. The doctrine of the Trinity enshrines the essentially Christian conception of God: it constitutes the ultimate evangelical expression of the Grace of the Lord Jesus Christ who though he was rich for our sakes became poor that we through his poverty might become rich, of the Love of God who did not spare his own Son but delivered him up for us all, for it is in that personal sacrifice of the Father to which everything in the Gospel goes back, and of the Communion of the Holy Spirit through whom and in whom we are made to participate in the eternal Communion of the Father and the Son and are united with one another in the redeemed life of the people of God. Through Christ and in the Spirit God has communicated himself to us in such a wonderful way that we may really know him and have communion with him in his inner life as Father, Son and Holy Spirit. (p. 2)
When I first encountered this paragraph I meditated on the first sentence since it seemed counterintuitive to me at the time. I wondered how it was that there could be a “pivotal center” to our knowledge of God that didn’t involve all three Persons if God is Trinity. But in wondering this I realized that the Lord Jesus Christ as the pivotal center is precisely what involves all three Persons! Jesus is the visible Person of the Trinity; the Incarnation ensured this. As such he reveals the Father (see John 14) but he doesn’t stop with revealing the Father he continues in dispensing the Spirit (Matt. 3:11; John 20:22). So often we think of revelation in terms of Scripture but the Incarnation of the eternal Son is by far God’s greatest means of self-revelation. The relational nature of the Trinity causes us to consider all three Persons whenever presented with any one in particular. In seeing the Son we see the Father; in receiving the Son we receive the Spirit. It was the revelation of the Son that most clearly revealed the Father and Spirit.