38 If you do not consider that I have now written enough in answer to your questions, you must have little knowledge of my capacities or of my engagements. For so far am I from being, as you have thought, acquainted with everything, that I read nothing in your letter with more sadness than this statement, both because it is most manifestly untrue, and because I am surprised that you should not be aware, that not only are many things unknown to me in countless other departments, but that even in the Scriptures themselves the things which I do not know are many more than the things which I know. But I cherish a hope in the name of Christ, which is not without its reward, because I have not only believed the testimony of my God that “on these two commandments hang all the Law and the Prophets;” but I have myself proved it, and daily prove it, by experience. For there is no holy mystery, and no difficult passage of the word of God, in which, when it is opened up to me, I do not find these same commandments: for “the end of the commandment is charity, out of a pure heart, and of a good conscience, and of faith unfeigned;” and “love is the fulfilling of the law.”
39. I beseech you therefore also, my dearly beloved, whether studying these or other writings, so to read and so to learn as to bear in mind what has been most truly said, “Knowledge puffs up, but charity edifies;” but charity vaunts not itself, is not puffed up. Let knowledge therefore be used as a kind of scaffolding by which may be erected the building of charity, which shall endure for ever when knowledge fails. Knowledge, if applied as a means to charity, is most useful; but apart from this high end, it has been proved not only superfluous, but even pernicious. I know, however, how holy meditation keeps you safe under the shadow of the wings of our God. These things I have stated, though briefly, because I know that this same charity of yours, which “vaunts not itself,” will prompt you to lend and read this letter to many.
Source: Letters (New Advent)