27 And now, Venerable Brethren, permit Us to close this letter by addressing to you these words of Moses: "If any man be on the Lord's side, let him join with me." We pray and entreat you to reflect on the great loss of souls due solely to ignorance of divine things. You have doubtless accomplished many useful and most praiseworthy works in your respective dioceses for the good of the flock entrusted to your care, but before all else, and with all possible zeal and diligence and care, see to it and urge on others that the knowledge of Christian doctrine pervades and imbues fully and deeply the minds of all. Here, using the words of the Apostle Peter, We say, "According to the gift that each has received, administer it to one another as good stewards of the manifold grace of God."
Source: Acerbo Nimis (Vatican.va)