"Why yes. Yes I do," I reply.
"So you just have 'blind faith' in the Church then?" he sighs, and mumbles something about "religious zealots."
Of course, the discussion shouldn't end here--there is more to the story. Rather than jumping to the conclusion of 'blind faith', one should rather ask why. Why do I believe X just because the Church says so? And once we know the answer, we can see that it is not blind faith at all; quite the opposite.
First of all, let's be clear on the scope of propositions that would call for such belief. For Catholics, it is propositions concerning faith and morals. The proper object of faith is God and, implicitly, our relationship to God and, further, our relationship to others in as much as it touches on our relationship to God, which is why morals are also considered, i.e., the practical application of our knowledge of God. I mention this to address people jumping to weird conclusions like if the Pope definitively declared the earth to be flat, I would believe it. I wouldn't, because it doesn't pertain to faith and morals (and because there is much evidence and other reasons to hold to the proposition that the earth spherical).
That touches on the second aspect to be considered, which is the interplay between reason and faith. Some jump to conclusions that faith is somehow inherently opposed or contrary to reason, and blind faith is the most irrational, unwarranted form of such "faith." But faith is not inherently irrational. Whether faith is irrational depends on whether the object of faith (the proposition) cannot be shown to accord with reason.
The reason I phrase it the way I did is that just because someone can construct a rational argument that leads to a conclusion opposed to the proposition does not exclude the possibility of one or more rational arguments that support it. And as long as there are rational arguments for a belief, it can be said to be rational. The proposition may not be true, but that does not preclude one's belief in it from being rational. The argument may not be compelling to you, but that does not mean it is not rational.
Up to this point, we've been speaking of objective rationality, that is, whether a rational argument exists for a proposition. There is also the subjective consideration. For instance, regardless of whether or not a rationale exists for a belief, if my belief were only based in emotion (or wishful thinking), in that sense my belief could be said to be irrational. In that sense the belief could better be described as prejudice or even blind faith.
So, back to the question at hand, is my belief in X that the Church proposes for assent by the faithful irrational? We must dig further.
One way to do this is to look for the rationale offered by the Church in proposing something for belief. The Church does not propose something for belief without offering some rationale for that proposal. In that sense, it would not be irrational to accept the proposition as1 true, because there is rationale, and I accept it because I find the particular rationale compelling.
Indeed, there is much in what the Church proposes, especially in terms of morals, that is discernable with reason without the aid of Divine Revelation, and it is in such things that we have so much common ground with non-Catholics. (There are also things the Church proposes based on the natural light of reason and not reliant on Revelation that we disagree on--such as abortion, redefining marriage, death penalty, etc.)
But my friend specifically qualified the question with "just because the Church says so." This is to say, do I accept the proposition (remember it is assumed to be concerning faith or morals) based only on the authority of the Church, without consideration for the rationale that the Church offers for it. That is the question.
In order for my assent to be rational, I would need some rationale to warrant that sort of trust in the Church, some reason to believe that the Church has the authority to define something as true and, consequently, to expect the assent of the faithful, even if they don't know the rationale, even if they don't fully understand it, even if they don't find the rationale convincing.
And that's precisely what I am suggesting--that the Church, which is the pillar and foundation of truth, has that authority, because it is granted and guaranteed by God, who is himself the very source of the truth, has the authority to define truths concerning faith and morals and that the faithful are bound to assent to them. (NB: There are degrees of certainty in truths proposed for assent, and there are corresponding degrees of assent required of the faithful, but that's a subject for another time.)
Now in this post, I'm not proposing to delve into all the reasons to believe that the Church is this pillar and foundation of truth. There are reasons, and it is through them that I was led to join the Catholic Church. But for now it is enough to simply point out that if we accept this as a given for the sake of this discussion--because for Catholics it is--then it follows, rationally, that we Catholics should assent to those things pertaining to faith and morals, even when we don't find the offered rationale for some particular teaching as compelling.