Catholic apologetics, much like the Catholic religion itself, is a mixed bag. When Catholics defend a Biblical idea, such as the Trinity, or the virgin birth, they tend to have no problem using the Bible. At the very least, they won't oppose other apologists who do so. This is even the case when their opponents misquote the Bible. Take, for example, "...my Father is greater than I" (John 14:28). This is a very popular verse to misquote when attacking the Trinity, to the point where every Trinitarian apologist should have an answer in his back pocket. And typically, Catholics do. They don't say "well, Church tradition says...". Their primary response is to put the verse back in context, and maybe return with a few other scriptures that show the Deity of Christ, and even the Trinity as a whole.
Why, then, is this not their attitude when their peculiar doctrines come under fire? When "Protestants" attack the Catholic Church, suddenly the ability to interpret the Bible goes out the window. They may claim their Church canonised the Bible. They may appeal to cherry picked Church "Fathers" who may or may not have agreed with their doctrines. They may even point out the large number of non-Catholic denominations (which they vastly exaggerate) and use it to suggest the Bible can only be sufficiently understood with reference to their authority. But getting them to focus on what the actual words on the page are is about as effective as explaining advanced economics to a child who has just suggested printing their own money.
This sorry state of affairs is very telling. For one thing, it tells us our common ground isn't so common. According to the Catholic Church, the Bible is to be revered equally to tradition, yet in practice, this is impossible. Indeed, going by the declarations of the Council of Trent, no one is permitted to interpret scripture in any other way than the Catholic Church does. Thus, rather than revering scripture and tradition equally, Catholics are required to revere Catholic interpretation and tradition equally. That is, they are to revere Catholic tradition equally to Catholic tradition.
But of course, you have to interpret Catholic tradition, right? After all, if there is some mystical veil blocking you from interpreting the teachings of the Catholic Church, you cannot be a Catholic. Yet this is exactly what the Catholic Church must claim is over the Bible, therefore making the Bible vastly inferior to Catholic tradition in Catholic practice.
And so the next question is, why? The Bible, after all, is a book. It is a collection of words intended to convey specific thoughts. If it was a Catholic book, surely it would read like one? Even if you want to argue there are multiple interpretations, so we need an authority to tell us which is correct, does it not make sense that a valid interpretation would at least fit the words on the page?
Yet, time and time again, we find the Catholic interpretation does not fit the words. Sometimes, this is a minor thing. Catholicism, for example, claims Mary remained a virgin her entire life, whereas the word "until" in Matthew 1:25 strongly suggests Mary and Joseph had a perfectly normal marriage, complete with appropriate marital activities, and later verses (as well as an earlier Psalm) show beyond reasonable doubt that this lead to other children. But this is such a minor error in the Catholic Church that we need not make a mountain out of a mole hill. Put simply, Mary's sex life is none of our business, and it's quite perverted that the Catholic Church would invent a whole dogma about it and bind it upon "all the faithful".
But one thing that can't be as easily forgiven, according to Galatians 1:8, is the fact that the Catholic Church preaches another gospel. According to Paul, if anyone does this, they should be considered accursed. The Gospel, according to the Bible, is very simple. We are saved by grace, through faith, not of works. One might summarise this as we are saved by faith alone.
The Bible even has an excellent example of this in the Penitent Thief. The Penitent Thief is a nightmare for the Catholic Church, as although he was promised salvation on the very day Jesus died, he did nothing the Catholic Church has added to salvation. He was not baptised. He did not receive the Eucharist. He didn't do anything He couldn't have done with his hands nailed to a cross. He was saved by faith alone.
But the Catholic "gospel" is a lot more convoluted than that. In truth, it is no gospel at all, for it offers no hope of salvation, either in theory or in practice. Cardinal John O'Connor even went so far as to say "Church teaching is that I don't know, at any given moment, what my eternal future will be. I can hope, pray, do my very best - but I still don't know."
But this brings us back to the original topic of scripture. When defending Biblical ideas, Catholics refer to scripture. When defending unbiblical ideas, they fall back on "Church teaching". "Sacred" tradition. Meaningless superstition. But this difference in attitude demonstrates that even Catholics recognise the difference in doctrine. If Catholicism was Biblical, they would not need to switch tactics to defend it. Indeed, they would never need to leave the Bible to at least put themselves on equal footing. The fact that Catholics must abandon scripture, whereas "Protestants" need never take a foot out of it, shows who the true Christians are, for Christ Himself said "...If a man love me, he will keep my words: and my Father will love him, and we will come unto him, and make our abode with him. He that loveth me not keepeth not my sayings: and the word which ye hear is not mine, but the Father's which sent me." (John 14:23-24).