Chapter II. The Unity Of The Church. 25 says St. Augustine, "that when it becomes necessary to defend certain points of Catholic doctrine against the insidious attacks of heretics they are more carefully studied, they become more [013] clearly understood, they are more earnestly inculcated; and so the very questions raised by heretics give occasion to a more thorough knowledge of the subject in question."29 Let us illustrate this. In the Apostolic revelation and preaching some truths might have been contained implicitly, e.g., in the doctrine that grace is necessary for every salutary work, it is implicitly asserted that the assistance of grace is required for the inception of every good and salutary work. This was denied by the semi-Pelagians, and their error was condemned by an explicit definition. And so in other matters, as the rising controversies or new errors gave occasion for it, there were more explicit declarations of what was formerly implicitly believed. In the doctrine of the supreme power of Peter, as the visible foundation of the Church, we have the implied assertion of many rights and duties which belong to the centre of unity. In the revelation of the super-eminent dignity and purity of the Blessed Virgin there is implied her exemption from original sin, etc., etc. So, too, in the beginning many truths might have been proposed somewhat obscurely or less clearly; they might have been less urgently insisted upon, because there was no heresy, no contrary teaching to render a more explicit declaration necessary. Now, a doctrine which is implicitly, less clearly, not so earnestly proposed, may be overlooked, misunderstood, called in question; consequently, it may happen that some articles are now universally believed in the Church, in regard to which doubts and controversies existed in former ages, even within the bosom of the Church. "Those who err in belief do but serve to [014] bring out more clearly the soundness of those who believe rightly. For there are many things which lay hidden in the Scriptures, 29 De Civitate Dei, Lib. 16, Cap. ii., No. 1.