[Alex Murphy]

If we grant that the early church was competent in declaring which books were Scripture then it was equally competent in declaring what was Christian doctrine. And if the church in the first centuries was competent to do this then it is so today.

You lost me with that last part. What "is so" today - the declarations of what is canon or doctrine, or the maintained competency allowing the modern church to make future declarations?

[Petrosius]

Either the church has authority to make declarations regarding both the canon of Scripture and regarding doctrine. One cannot exist without the other for the declaration of the canon of Scripture is in itself a question of doctrine. Now if this authority existed in the past it must continue to exist today.