Agree using what means? Your own intellect and historical criticism? Again, how are you going to determine if the Johannine comma is part of the canon or not? If it is not in the earliest manuscripts, should we include it in the Bible or not? Catholics know the answer, do you?...
The 100,000 dollar question is who is going to be the authority that is going to decide what historical account is correct? Who is going to decide what manuscripts are the closest to the original? You have to answer the question about the Johannine Comma. Is it part of the canon or not. If or if not, why?
I've seen a lot of Catholics make unreasonable claims about an alleged need for an infallible church to tell us what to believe. But Matthew Bellisario's use of the Johannine Comma is one of the worst examples I've encountered. Those of you who are familiar with the evidence pertaining to the Johannine Comma should know that it's not difficult to judge whether the passage is authentic. And even if it was difficult to judge, we'd make a decision in the same manner in which we make decisions about the text of patristic documents, papal documents, conciliar documents, etc. We don't need a ruling from an infallible church to arrive at a reliable conclusion.
"Brethren, do not be children in your thinking; yet in evil be infants, but in your thinking be mature." (1 Corinthians 14:20)