John Chrysostom

"That good thing which was committed unto thee keep,"-how?-"by the Holy Ghost which dwelleth in us." For it is not in the power of a human soul, when instructed with things so great, to be sufficient for the keeping of them. And why? Because there are many robbers, and thick darkness, and the devil still at hand to plot against us; and we know not what is the hour, what the occasion for him to set upon us. How then, he means, shall we be sufficient for the keeping of them? …

Homily on 2 Timothy 3 ↗

Theophylact of Ohrid

Keep what I have commanded you regarding faith and life, or the gift which you received, as he also said above. But how will you keep it? Not by human strength, but by the Holy Spirit, Who dwells in us through baptism. If we preserve Him and do not drive Him away by evil deeds, He too will preserve for us all that we have from God. Therefore, strive to preserve the Spirit, and He will preserve for you what has been entrusted. "Unless the Lord guards the city, the watchman stays awake in vain" (Ps. 126:1).

Commentary on 2 Timothy ↗

Thomas Aquinas

Therefore, having these things, keep the good thing committed to your trust, namely, which I have entrusted to you, namely, the office of preaching, and never depart from the truth or give up the office of preaching at any time because of fear: with all watchfulness keep your heart (Prov 4:23); O Timothy, keep that which is committed to your trust (1 Tim 6:20). And keep this with the help of the Holy Spirit who dwells in us: know you not that you are the temple of God, and the Spirit of God dwells in you? (1 Cor 3:16). …

Commentary on 2 Timothy ↗