Why is it important to believe God is Trinitarian? Because ONLY THE Trinitarian God is able to save. Unless you have a Trinitarian God, there can be no salvation; without a Trinitarian God there is no atonement for sins. How could God take our sins upon him and remain pure, even when he has not sinned. How could the judgment of the Father be upon the Son and our sins if our God was not three persons but one God? How could we be filled with the Holy Spirit and be adopted as his sons and daughters unless God was trinity? How could God have been born as a human being, emptying himself of his divinity, and still remain sovereign over the universe? How could God know what love is and how to love if he has spent eternity alone before creating the world? Only a God who has been in a perfect eternal relationship in the trinity can know and then create in his image beings who can love.
God has to be Trinitarian to be the God who loves us and who has rescued us from sin. No other God is able to save.
This is why our proclamation, our witness of God is in aTrinitarian pattern:
The Father Sends the Son to the earth; then the father and the Son send the Holy Spirit; and now, here in Matthew, the Father, Son and Holy Spirit send the CHURCH into the world to make disciples in the Trinitarian formula, in the name of the father, the son and the holy spirit.