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).
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).