Once upon a time there was a God who wanted to be worshipped by everyone.
HE chose the descendants of Abraham as his vehicle to enter into the minds of everyone in the world.
HE gave Moses the LAW, the first and foremost being that Moses people should not worship any other gods on pain of death.
Moses people believed God and would put any of their people to death for not worshipping the one true God.
When Jesus preached that he was the Son of God and allowed people to worship him, the priests of God convicted Jesus of blasphemy and had him killed.
The people who worshipped Jesus said that the priests who had Jesus killed were wrong because God had just forgotten to mention that He was more of a 'three-in-one' God.
God tricked the descendants of Abraham's people into thinking that HE was just the one God when HE was actually a 'three-in-one' God and used HIS chosen people to sacrifice the Jesus part of HIMSELF to show that HE was not just God of the Jews, but God of everyone.