How can God be a Trinity? There can’t be three Gods, so where did the doctrine come from? Some nit-picking council of ancient theologians and bishops perhaps? No.
Throughout the Old Testament humans were confronted by the vast mystery of the God, who created them, and the whole universe. He was a God who knew their names and who was in some way personal. Like a father in fact, but beyond human touch and testing. He apparently wanted people to know Him. He came to them: told them who He is, the great ‘I AM’, who actually loved people and is the source of justice and truth and awesomeness. Then to those very folk who knew him as father and king, invisible and mysterious, there came another massive set of facts.
A young man strode onto the world scene, and the middle-eastern world beat a track to his obscure and rural door. He did things only God has the power to do: said the things only God has the right to say: he was awesome, and humble. He spoke of his Father beyond, from whom he came. The big issue became WHO IS HE? Those who saw him, heard him, lived with him, and had opportunity to detect any fanaticism or self centredness: called him ‘King’ – ‘Lord’ – ‘Son of God’ – ‘GOD’.
This was no super Jew. This was someone from beyond our sinful race which murdered him, someone uninfected by the human plague of ego-mania. Then when he had left their sight, it happened – what he had promised – something massive happened. No, someone happened.
Those who trusted in Jesus and His Father were able to do and say and be the very things which were of the same order as the God they trusted. The only way they could make any sense of it was that it was the same God, who was now flooding and working inside them. The Spirit was the same God, but now inside their skins. It was all beyond their little logic, but known to them by faith and massive, undeniable experience.
“This is what was spoken by the prophet Joel: I will pour out my Spirit on all people.” Acts 2: 16, 17a.

A Prayer:
Lord, I try to grasp the mystery of one God in Trinity by picturing water, as liquid, as ice and as steam: but in the end, bow before the person of the Father God above me, the Son who confronts me, and the Spirit of God within me, and marvel at the mystery which is beyond my little logic.

Now read Isaiah 9: 2-7