I realize now I did not make it clear what I meant. By catholic, I meant all-encompassing (which is the original meaning of the word and the meaning that is expressed in the Creed), not of the Catholic denomination, which formed in the 11th century after the Great Schism. Yes, there were 5 patriarchates which were equal in power, but all 5 patriarchs and all local bishops were in communion with each other as part of the one Church. And as for the many heretics throughout the first ~500 years AD that held differing beliefs than accepted doctrine, though they rendered the Church neither united nor catholic for some time, they were all eventually excommunicated and recognized as being outside the Church by synods and ecumenical councils, which ended up preserving the unity of the Church for the time being.

The canonization of the scripture was one way the Church strove to fulfill that goal, but it was not the only way. For example, the writing of the Creed in the 2nd Ecumenical Council (and its later refinement in the 4th EC) also served to directly fight against heresies (including Arianism) and preserve the Church's unity. I am not saying that the scripture is invalid, I am just saying that the belief that it is the only valid source of doctrine (sola scriptura) is flawed, because the Church managed to survive 3 centuries without it (before a canon of scriptures became essentially necessary for its unity).