We can be reasonably sure in the same way we are sure of anything. Presumably you are asking for some special, infallible, miraculous certainty. We don’t have this about anything so why about Scripture? God hasn’t seen fit to make the world that way.
No infallible authority has told me that my wife loves me, or that my parents are really my parents, or that I was really born in the city of Blackburn on May 13, 1974. Yet I firmly believe all these things.
The question is an illegitimate one from the start.
Never mind that (as Protestant apologists have pointed out over and over to no avail) Catholics are in the same boat. You believe in the infallible Magisterium either because the Magisterium says it is infallible (logically prior to your decision that it is infallible, and hence inadmissible as an “infallible authority”) or because you have concluded this on the basis of the historical, Biblical, experiential (etc.) evidence, which is exactly how Protestants interpret Scripture. And then there’s the fact that the Magisterium’s statements need to be interpreted, and that this applies ad infinitum to the Magisterium’s explanations of its own statements (even assuming that all such statements were infallible, which they aren’t).
I have seen this line of reasoning often mocked, but never refuted. The degree of certainty you are demanding for the interpretation of Scripture (i.e., verification by an infallible authority so that it cannot possible be doubted) is impossible. It is logically incoherent and leads to total skepticism.
There are plenty of fatal problems with Protestantism (fatal to any claims it might have to be a self-standing version of Christianity, that is) without this. You don’t need to embark on half-baked epistemology in order to defend Catholicism or even (which is not the same thing) attack Protestantism.
In Christ,
Edwin