We recently celebrated the Feast of the Holy Trinity. On this grand solemnity, the Church once again, reminds its faithful of the truth that God is in three persons, the Blessed Trinity. The Council of Nicea in 325AD was convened predominately because the doctrine of the Trinity was not being understood among the Church. Despite the gospels and letters of the apostles being circulated among the Churches, almost half of Christendom was believing that Jesus was not truly God.
It took a council of the Catholic Church, not just personal interpretation of Scripture, to authoritatively state that God was indeed three persons: Father, Son and Holy Spirit. Scripture alone without the authoritative voice of the Church, was not enough.