Theophylact of Ohrid

Having finished the speech conducted above, the apostle adds concerning those who deceive them with heresies that have appeared in great number. Then, in order to remove grief from them, he again repeats: "and the anointing which you received from Him." By anointing, as already said, is understood the Holy Spirit. Therefore, if you firmly preserve within yourselves the Holy Spirit, whom you received, then you have no need for anyone to teach you. …

Commentary on 1 John ↗

Oecumenius

I have written these things to you about those who are trying to deceive you. And as for you, the anointing that you received from him remains in you. And you do not need anyone to teach you, but as his anointing teaches you about all things, and is true and is not a lie, just as it has taught you, remain in him. "Truly, concerning these things", John adds: "I have written to you about those who deceive you": that is, because of the heresies that have emerged. …

Commentary on 1 John ↗

Augustine of Hippo

"These things have I written unto you concerning them which seduce you; that ye may know that ye have an unction, and the unction which we have received from Him may abide in you." In the unction we have the sacramental sign of a thing unseen, the virtue itself is invisible; the invisible unction is the Holy Ghost; the invisible unction is that charity, which, in whomsoever it be, shall be as a root to him: however burning the sun, he cannot wither. All that is rooted is nourished by the sun's warmth, not withered.

Ten Homilies on 1 John 3 ↗

Bede

These things I have written to you about those who deceive you. By deceivers whom he names, not only are heretics to be understood, who seek to turn away from the faith by wicked doctrine, but also those who, by allurements or by adversities of the world, detract the minds of the weak from the promise of eternal life, either by enticing them evilly or by frightening them.

Commentary on the Catholic Epistles ↗