A Catholic Apologist is saying that if the Church teaches something about it's own infallibility then that is the guarantee that it is true. It would seem, at least superficially to be a circumlocutory style of argument. Repeating the claim does not improve its veracity or otherwise, nor does it help me to accept it.
Well, Catholics argue that Scripture establishes her as The One True Church® but that one cannot assuredly know what Scripture (etc.) consists of and means apart from faith in her (esp. after they fail to convince one that Scripture supports her distinctive claims).
People cannot discover the contents of revelation by their unaided powers of reason and observation. They have to be told by people who have received in from on high. - Cardinal Avery Dulles, SJ, "Magisterium: Teacher and Guardian of the Faith, p. 72;
...the believer cannot believe in the Bible nor find in it the object of his faith until he has previously made an act of faith in the intermediary authorities..." - Catholic Encyclopedia>Tradition and Living Magisterium
And as Keating basically expressed, which faith in her claims is warranted since she possesses ensured (if conditional) magisterial infallibility, since she has infallibly declared that she does.
But many will also attempt to basically argue that being the instruments, discerners and stewards of express Divine revelation, and the inheritor of promises of God's presence and guidance means or requires that such possess said ensured infallibility. But which necessity is not seen in Scripture and effectively invalidates the NT church.
They may also argue that their reasoning is "spiral," that rather than needing to have faith in the Church in order to see what Scripture means and thus place faith in=submission to the Church, they will appeal to Scripture as merely a reliable historical document that provides evidence which warrants said faith and submission, that of a fallible decision in an infallible authority.
However, this is still contrary to the basic premise that one needs to believe the Church to know what is of God, otherwise they must allow that faith in her is not needed for souls to know what is of God, and the are frustrated when souls do not find their arguments convincing.
However, the church began with souls having assuredly discerned what was of God, both men and writings, without an infallible magisterium to tell them, and even in dissent from the magisterial stewards of Holy Writ, and inheritor of the promises of God. (Mt. 23:2; Romans 3:2; 9:4,5)
