How did the Trinity got decided?

When Constantine converted to Christianity he had no interest into this theological discussion. Nor did most of the early Christians. However most of the bishops in Alexandria (considered the cultural centre of the empire) were Arian, meaning they believed Jesus was somehow subordonate to the Father. But the head of Alexandian branch did not and Constantine had to intervene.

The council of Nicaea in 325AD was called to settle the issue. Based mostly on politics the Aerian position lost and it was decreed that God and Jesus are of the same substance.

100 years later than the Holy Spirit joined.