“So, tell me. Upon what do you base your Christianity?”
Since you’re late to the discussion, please start by answering this question: Is a Catholic who has faithfully followed the Catholic faith and its doctrines for his entire life a Christian?
No, he is a Catholic. It is a habit of Catholics to equate Christianity with Catholicism. Do you base your claim to Christianity upon the Word of God or upon the Traditions of Man or both? Catholics follow the teachings of the church, therefore they are called Catholics. Christians follow the teachings of Christ as revealed in the Word of God. What is your final authority? The Church or the Book? That will answer the question you asked of me.
One can be a Baptist, Catholic, Methodist, Episcopalian, etc. and not even be remotely Christian. It boils down to do we follow Christ. Are we Christ-like? I seem to remember participating in a never-ending thread (I assume it eventually ended) here on FreeRepublic many years ago that was Bible-Believers versus Catholics. I’m not too interested in starting that back up again.
With regards to final authority, The Lord can say it so much better himself anyway:
These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.
1 John 5:13
It is the Word in which we find info pertaining to eternal life, not in a man-made institution such as the Catholic Church, Inc.