The Holiness of the Church as a Motive of Credibility and the "Argument from Meager Moral Fruits"

Let your light so shine before men, that they may see your good works and give glory to your Father who is in heaven — Matthew 5:16

In Catholic apologetics, there are various motives of credibility that the apologist can appeal to in his defense of the Faith. The twentieth-century Thomist theologian Fr. Réginald Garrigou-Lagrange, in his masterwork on fundamental theology and apologetics, defines and explains motives of credibility as follows:

According to the Catholic Church, the motives of credibility are the signs or notes by which revealed religion is made evidently credible to divine faith. They are called signs or notes inasmuch as they manifest the divine origin of revealed religion (or inasmuch as they are "arguments for divine revelation"). They are called motives in relation to the judgement of credibility, which is founded on them" (On Divine Revelation: The Teaching of the Catholic Faith, I.16.1.3; emphases in original).

One motive of credibility in particular is the holiness of the Church, particularly with respect to her many extraordinary saints and martyrs as well as the good works and general virtuousness of her ordinary practicing members (the latter will be my focus in this post). The Church herself draws attention to this motive of credibility in her official teaching:

All Have Sinned? On an Argument Against the Sinlessness of Mary

A standard Protestant argument (and the one that seems to be the most popular on social media) against the Catholic doctrine of Mary's sinlessness is rooted in the alleged fact that Sacred Scripture teaches that all human beings have sinned:

[A]ll have sinned and fall short of the glory of God (Romans 3:23).

Now, since Mary is a human being, it follows that Mary is a sinner. Apparently, then, the argument for Mary being a sinner is as follows:

1. For any x, if x is human, then x is a sinner.
2. Mary is human.
3. Therefore, Mary is a sinner.