13 “And you have no need that any man teach you, because His unction teaches you concerning all things.” Then to what purpose is it that “we,” my brethren, teach you? If “His unction teaches you concerning all things,” it seems we labor without a cause. And what mean we, to cry out as we do? Let us leave you to His unction, and let His unction teach you. But this is putting the question only to myself: I put it also to that same apostle: let him deign to hear a babe that asks of him: to John himself I say, Had those the unction to whom you were speaking?
You have said, “His unction teaches you concerning all things.” To what purpose have you written an Epistle like this? What teaching did “you” give them? What instruction? What edification? See here now, brethren, see a mighty mystery. The sound of our words strikes the ears, the Master is within. Do not suppose that any man learns ought from man. We can admonish by the sound of our voice; if there be not One within that shall teach, vain is the noise we make. Aye, brethren, have ye a mind to know it?
Have ye not all heard this present discourse? And yet how many will go from this place untaught! I, for my part, have spoken to all; but they to whom that Unction within speaks not, they whom the Holy Ghost within teaches not, those go back untaught. The teachings of the master from without are a sort of aids and admonitions. He that teaches the hearts, has His chair in heaven. Therefore says He also Himself in the Gospel: “Call no man your master upon earth; One is your Master, even Christ.” Let Him therefore Himself speak to you within, when not one of mankind is there: for though there be some one at your side, there is none in your heart.
Yet let there not be none in your heart: let Christ be in your heart: let His unction be in the heart, lest it be a heart thirsting in the wilderness, and having no fountains to be watered withal. There is then, I say, a Master within that teaches: Christ teaches; His inspiration teaches. Where His inspiration and His unction is not, in vain do words make a noise from without. So are the words, brethren, which we speak from without, as is the husbandman to the tree: from without he works, applies water and diligence of culture; let him from without apply what he will, does he form the apples?
Does he clothe the nakedness of the wood with a shady covering of leaves? Does he do any thing like this from within? But whose doing is this? Hear the husbandman, the apostle: both see what we are, and hear the Master within: “I have planted, Apollos has watered; but God gave the increase: neither he that plants is any thing, neither he that waters, but He that gives the increase, even God.” This then we say to you: whether we plant, or whether we water, by speaking we are not any thing; but He that gives the increase, even God: that is, “His unction which teaches you concerning all things.”
Source: Homilies on the First Epistle of John (New Advent)