The Catholic Church claims to be the true Church. She claims, as do the Orthodox, to have not changed the doctrine of the Church since Jesus first gave her the deposit of faith. The initial conflict between her and the Orthodox east came when she changed the Nicene Creed, which the entire church had agreed upon, from “The Holy Spirit who proceeds from the Father” to ” The Holy Spirit who proceeds from the Father and the Son”. The Roman Church admits to changing the Creed, but maintains that the change did not alter the faith.
To begin with, a change in the doctrine of the Trinity cannot be considered to be an insignificant change. In fact, the nature of the God you are worshiping is the first and most important thing to get right. That being said, it is either true that the Holy Spirit proceeds eternally solely from the Father, or that he proceeds eternally from both the Father and the Son. Either the entire church was wrong when she wrote the Creed, and therefore left the faith for approximately 700 years, or the Roman Church has been in error for approximately 1,000 years. Both cannot be right as the Catholic Church now claims. One must be right and one must be wrong.