How do you know that you are who you are? You’re parents (claim) to have given you a name, they’ve been throwing you a party each year (I hope) on the same day and calling it your birthday. When you’re old enough, the state gives you a little plastic card with that same name and same birthdate. And, if you lose that little plastic card, you can use a piece of paper they (suppopsedly) issued your parents at your birth that says basically exactly the same stuff as both your parents and that little plastic card say.
Of course you believe this, but even if you didn’t, even if you had the mother of all existential meltdowns and wanted to track down the doctor who delivered you, the nurse who washed you up, the neighbor who saw your folks bring you home from the hospital- you can’t take it as “proof”. Even if there was a photograph of you as an infant with your parents, there’s still room for doubt- it could be a different baby for all you know. Is it reasonable evidence? Sure. Because either its true, or you’re living in the Truman Show and everyone is conspiring against you. Because, hopefully, your parents don’t strike you as the type of people who would lie to you about something like that (maybe about Santa Clause, but not about your own birth). So there’s a point where we have to acknowledge the reasonable, logical limit of the evidence available to us.
What we’re doing by citing things like the Early Church Fathers’ writings is the equivelent of showing you our drivers license and birth certificate. You can logically make the claim we’ve all been duped or misled, but in doing so you’d be ignoring some pretty strong evidence. None of us claim the Catholic Church of today is absolutely identical to the ancient church, and there’s no branch of science, human thought, culture, even language that could boast a similar conviction. What we are saying is the Apostles are equivelent to our parents naming us and telling us when our birthday is. Scripture, the Apostolic writings and oral Traditions, these are our identifying documents we hold up as evidence we are who we say we are (Catholics). Its as much proof as any of us, yourself included, can go by since its impossible to verify with 100% certainty. And if we consider the Canon to have been closed by these same men, to recognize their authority in one case and not in another is close to agreeing with your parents that you were born, you just don’t believe the date they claim you were born on is correct.