The Bible says that there is one God. If you are a Christian don't insult Jewish people by trying to explain Trinity if you personally don't understand 'The Economy of God' and the concept of the Mosaic High Priest. Also, if you are a Gentile (you will be hardwired with the mind of Athens - not Israel: so that the existential notion of ‘lifeworld’ may be better suited to your understanding. Using this philosophy God entered our lifeworld through Messiah in order to die as a full payment sacrifice (outside that lifeworld beyond the space-time-continuum, for example, God cannot die as he is all powerful).
The Messiah is the Triune God, as is the Spirit and the Father, they are one and the same. To properly believe in Jesus is to believe in one God.
An important clue as to the necessity of Messiah from the Old Testament, is that God had a mediator under Mosaic Law. This is because a person could not approach God under the sacrificial system: instead had to approach Him by means of a chosen mediator the Jewish High Priest.
Similarly as the Old Testament believer had to go through the High Priest, the New Testament believer must go through the High Priest Yeshua HaMashiach: Jesus the Messiah.