Often Protestant apologists assume that the Bible is formally sufficient; this is a false a priori assumption. Mark Shea, a Catholic apologist, wrote the following on the difference between “material” and “formal” sufficiency; it shows that, even if one allows that the Bible is “sufficient,” it is simply question-begging from the Protestant that this translates out into sola scriptura/the formal sufficiency of the Bible:
[T]here is a difference between material and formal sufficiency? Simply put, it is the difference between a big enough pile of bricks to build a house and having a house made of bricks. Material sufficiency means that all the bricks necessary to build doctrine is there in Scripture. However, it also teaches that since the meaning of Scripture is not always clear and that sometimes a doctrine is implied rather than explicit, other things besides Scripture have been handed to us from the apostles: things like Sacred Tradition (which is the mortar that holds the bricks together in the right order and position) and the Magisterium or teaching authority of the Church (which is the trowel in the hand of the Master Builder). Taken together, these three things--Sacred Scripture, Sacred Tradition, and the Magisterium--are formally sufficient for knowing the revealed truth of God.
In contrast, those who hold to Bible-only revelation hold the notion that Scripture alone is formally sufficient and therefore does not need Sacred Tradition or the Magisterium to elucidate its meaning. The idea of the formal sufficiency of Scripture has been asserted in various ways, but all the attempts to do so depend on a confusion. Typically, there is the persistent idea [despite the overwhelming biblical problems with sola scriptura] that the "word of God" refers simply and solely to Scripture. Thus, in argument after argument, those who hold to the formal sufficiency of Scripture warn darkly that setting Scripture in the context of Sacred Tradition will inevitably put Scripture under the Church. The fear, in fact, is that to admit the revelatory nature of Sacred Tradition will necessarily subjugate Scripture to merely human agendas. For despite the biblical evidence that Tradition can sometimes be authoritative revelation, there remains the lingering notion that, when all is said and done. Scripture alone is the basis of the Church in the same way, for example the Constitution of the basis of the United States of America. In short, the foundation of the Church is the word and the word is the Bible. Any appeal to Tradition is therefore seen as an attempt to move the House of God off its foundations. (Mark Shea, "What is the Relationship Between Scripture and Tradition" in Robert A. Sungenis, ed. Not By Scripture Alone: A Catholic Critique of the Protestant Doctrine of Sola Scriptura [2d ed.; Catholic Apologetics International Publishing, 2013], 155-91, here, pp. 165-66; comment in square bracket added for clarification)
