Despite all the difference between Protestants and Catholics, and there are many, Catholics are still Christians. Period. A lot of issues that Protestants disagree with Catholics, such as praying for the dead, purgatory, veneration of Mary, etc., are based on the rejected books. The Catholics are not pulling this out of thin air. Martin Luther took at least seven books from the Catholic Bible, and wanted to also take out Revelation, Jude, James, and Hebrews. The Catholics have their reasons on why they regard these books highly/ same level as Scripture, to me the reasons are logical and supported by the books they use. The Catholics give heavy weight to the Catholic Church/tradition because the Catholic Church put together the books of the Bible (particularly New Testament) and so they have some authority, or did the Bible put together the Catholic Church, it is the chicken or the egg argument.
The early Catholic Church put together the Canon (the books in the Bible, before Martin Luther took out some of these books for the Protestants).