Sacred Tradition

Protestant: The Bible is very clear, it is also complete, nothing else is needed.

Catholic: Maybe, except that the Bible itself clearly states otherwise.

For instance, where in the Bible does it show the list of books that are inspired, and which, therefore, should comprise it? If nowhere, then how was its canonicity established? By Catholics? But how would Catholics have been able to? There's only one answer: Sacred Tradition, which is Christ's teaching preached- exactly how Christ commanded it to be.

What is your doctrine on God the Son in relation to God the Father? Is the Son consubstantial with the Father? Who came first: the Father or the Son? Or did neither of them come after the other, both having no beginning? Can you cite passages which support your doctrine?

What is your doctrine with regard to the Son? Does he have two natures- one human, the other divine—or only one? Does he have two persons- one human, the other divine- or only one? Where in the Bible can your answer be found?

Is the Catholic teaching about the Holy Spirit proceeding from the Father and the Son correct or not? Can you show me where in the Bible can your answer be found?

Who wrote Matthew? What Bible passage made you say that?

You will find that you cannot answer the above questions merely by referring to the Bible. Which is why Protestants should follow what the Bible says, and not disown Sacred Tradition. There are things in the Bible that only Sacred Tradition can illuminate.