In Introduction to Christianity, Joseph Ratzinger explains how the dogma of the Trinity emerged from early Christian experience.
The apostles discovered that “in Jesus Christ one meets a man who at the same time knows and professes himself to be the Son of God. One finds God in the shape of the ambassador who is completely God and not some kind of intermediary being, yet with us says to God ‘Father'” (115).
Ratzinger calls it a “paradox”: “one the one hand this man calls God his Father and speaks to him as to someone else facing him; if this is not to be a piece of empty theatricality but truth . . . then Christ must be someone other than this Father to whom he speaks and to whom we speak.” At the same time, “he is himself the real proximity of God coming to meet us, God’s mediation to us,” and this is so because “he himself is God as man, in human form and nature” (115). In Jesus, “God meets me . . . not as Father but as Son and as my brother,” and by this a “duality appears in God: God as ‘I’ and ‘You in one.”
Arianism makes a hash of this. If Jesus isn’t God, then “his mediation would . . . basically cancel itself out and become a separation instead of a mediation.” If He is not God-with-us, God’s proximity coming to meet us, then “He would . . . be guiding us not towards God but away from him” (115).