In the second story of the first day of the Decameron, we have the story of how Giannotto tried to convince a Jewish friend named Abraham to become a Christian. Giannotto is a fairly ignorant merchant, but his arguments have sincerity. Abraham, on the other hand, is a theologically well-educated Jew. But instead of making mincemeat of his friend's arguments, out of friendship and perhaps a movement by the Holy Spirit (so the narrator suggests), he resolves he'll go to Rome to see what the alleged vicar of Christ is like, in order to decide which faith is correct. Giannotto thinks all is lost:
if he goes to the court of Rome and sees the wicked and filthy lives of the clergy, not only will he not change from a Jew to a Christian, but if if he had already become a Christian before, he would, no doubt, return to being a Jew.Nonetheless, he sends his friend with his blessing. Abraham goes to Rome and sees all the sin among "the Pope, the cardinals, and the other prelates and courtiers". Abraham returns, and Giannotto is sure that there is no longer a chance of conversion. He asks Abraham what he thought of the Papal court. Abraham responds:
I don't like them a bit, and may God condemn them all; and I tell you think because as far as I was able to determine, I saw there no holiness, no devotion, no good work or exemplary life, or anything else among the clergy; instead, lust, avarice, gluttony, fraud, envy, pride, and the like and even worse (if worse than this is possible) were so completely in charge there that I believe that city is more of a forge for the Devil's work than for God's: in my opinion, that Shepherd of yours and, as a result, all of the others as well are trying as quickly as possible and with all the talent and skill they have to reduce the Christian religion to nothing and to drive it from the face of the earth when they really should act as its support and foundation. And since I have observed that in spite of all this, they do not succeed but, on the contrary, that your religion continuously grows and becomes brighter and more illustrious, I am justly of the opinion that it has the Holy Spirit as its foundation and support, and that it is truer and holier than any other religion.... So, let us go to church, and there, according to the custom of your holy faith, I shall be baptised.[note 1]
Now, while over the past century we've been blessed by popes of exemplary holiness (though of course there has been much wickedness elsewhere among the clergy and laity), the argument does not require present papal wickedness. What it requires is the surprising way that despite all the wickedness, the Church survives and grows. One might object: but if the Catholic faith were the true faith, wouldn't we expect that the hierarchy would be holy in the first place? While the analogy is not perfect, this is similar to asking, in the case of someone who was apparently miraculously healed, why God would have permitted the illness in the first place. The question is a good and tough one, but it does not make the healing (in the case of the cancer) or the survival and growth (in the case of the Church) less wonderful.
We might enhance the above by recalling another argument. The Catholic Church's formal teaching is coherent, despite having been developed over twenty centuries. The teachings are not only coherent at one time, but are coherent over time (and cohere with Scripture as well, but I don't want to rely on this if the argument is to be convincing to Protestants). The best explanation of this coherence is that it is the work of the Holy Spirit. Arguments along these lines have been developed by Menssen and Sullivan. Observe, too, how this consistency is not observed in most other Christian bodies—sexual ethics is a nice example, with contraception once condemned by all theologians (including Luther and Calvin) and now widely accepted by non-Catholic bodies (with the notable exception of some individual Protestants and some Orthodox bodies—though even in the latter, there is a reluctant acceptance of remarriage after divorce). But now combine this argument with Boccaccio's. The consistency over time is amazing enough—but when one notes that the consistency includes popes who were, apparently, quite wicked, but who, nonetheless, did not formally teach the Church anything contrary to the earlier faith, the argument becomes even stronger.