Just a quick question - if I can give a consistent, Scriptural answer to the question that a Protestant can’t answer, what do I win?)
Well, anyone can give a consistent Scriptural answer, that ain’t the problem. The problem for you, if you believe in salvation by faith alone, is that you cannot give an answer that is consistent with your theology and with Scripture at the same time.
Here’s some food for thought - Do you “infallibly’” understand the “infallible” teachings of the Catholic Church, or are you relying on your own, fallible interpretation of what you believe is being taught? Seems to me you are in the same boat I am, except that I recognize my need to study the Scriptures to better understand what God has for me (not to mention that my source (the Scriptures) do not “develop” new doctrines over time, unlike your source, but that would be another topic).
Sorry, but your premise is flawed. You do indeed have only your fallible interpretation to rely upon when you interpret the Bible because the Bible cannot speak. The Bible cannot say to you, “Excuse me, but that’s not what those words mean.” So, you are stuck listening only to the voice in your head that says, “I can read the Bible, therefore I am Pope, pastor, and theologian of the Church of Cachonga and will decide for myself, by my own authority, what is true doctrine and what is false doctrine.”
I, on the other hand, have an infallible authority that can actually talk to me, that can teach me, that can correct me when I err and point me to the truth. I have a teacher that is the pillar and ground of the truth. Do you have such a teacher? I have a teacher who can say, “Whoever listens to us is of God, and whoever does not listen to us is not of God. By this we know the Spirit of truth and the spirit of error.” Do you have such a teacher?
So, no, we are not in the same boat. You are quite right that your “source” cannot develop new doctrines, it cannot do anything except be rightly or wrongly interpreted. However, neither can my “source” develop new doctrines. It can, however, teach the truth entrusted to it by Jesus Christ through His Apostles. It can also correct those who believe and teach error. My source is infallible when teaching on faith and morals. Your source is only as infallible as the person reading and interpreting it. It is the inspired Word of God, and it is inerrant, but what guarantee of doctrinal soundness does it offer to the ignorant and the unstable when they interpret it in a fallible way and by their own authority?