Since God has no physical form, and we are expressly told not to think of Him as having human shape (Deuteronomy 4:15,16), how are we to imagine Him? As some kind of amorphous blob? Surely that would reduce Him to a mere philosophical concept. But the doctrine of the Trinity suggests something (someone) far more interesting and energetic: the Father, Son and Spirit interlocked in an eternal triangular embrace, each loving (and being loved by) the other two, each giving continuously of themselves for the benefit of the others, and each rejoicing in the company of the others. This God is “a dynamic, pulsating dance of joy and love.” (T Keller)
So a simple one-sentence definition of God could be “Father, Son and Spirit loving one another.” We have a God who is neither singular nor plural, but One-in-Three and Three-in-One.None of the three Persons of the Trinity stands alone; they are bound together by love. There is a reciprocal relationship between the Father and the Son (John 14:9): Jesus glorifies the Father,and the Father glorifies Jesus (John 17:1-5).
Yet there is no blurring of their roles, and they are not interchangeable. It is the Father who is above all, and He is the prime Mover who always initiates. The Son then executes the will of the Father, and the Spirit provides the power with which the Son operates. The Son was sent by the Father (John 8:42); it was the Son (not the Father or the Spirit) who became human and died on the cross. The Holy Spirit in turn is sent by the Father (John 14:26) and by the Son (John 16:7); He is the both the Spirit of God and the Spirit of Christ (Romans 8:9).
So, for example, the Father makes Himself known through His Word – the Son (Matthew 11:27; John 1:18). The Son is then made known to us through the Spirit-breathed Scriptures (John 5:39,40). For the main function of the Holy Spirit is to make Christ known (John 16:14). This sheds some light on Jesus’ rather curious (and much misunderstood) statement about blasphemy against the Holy Spirit (Luke 12:10). Those who blaspheme against the Father or the Son may be forgiven because they are still open to the Holy Spirit. He can still reveal Christ to them, convict them of sin, and bring them to repentance and faith. But if someone turns against the Spirit, there is nothing and no-one left who can restore them to the truth; they have cut themselves off from the Father and the Son as well.
What does the Trinity doctrine teach us? That God is relationship; God is fellowship. His nature is to want fellowship with us (I John 1:3) – and since we have (as a species) repudiated Him, what He desires above all is reconciliation (II Corinthians 5:18-20). This is why peacemakers are like Him (Matthew 5:9).