"This charge, or adjuration, suggests three thoughts. The command to preach the word is accompanied by fearful consequences for those who do not fulfill it. Its fulfillment is urgent for those upon whom it is laid. The word commanded to be preached is the only saving word. For if it were not such, it would not be commanded to be preached with such insistence. Moreover, this suggests that the preacher does nothing extraordinary, but fulfills his duty if he preaches with all zeal, without yielding to the usual sloth in this matter" (paraphrase of Oecumenius).
"Before God and our Lord." That is, before God the Father and the Son of God, Who became incarnate for our salvation. When reading this, do not admit even the slightest thought of inequality, not even by an imperceptible line. Nor think that these two Persons ever exist, in anything, without the third, the Holy Spirit. They are always inseparable.
"Who is to judge." "Is to" means "is going to." "In the First Epistle he inspired Timothy with fear by the words: I charge you before God, Who gives life to all things (cf. 1 Tim 6:13). Here he says something more fearful: Who is to judge, that is, Who will demand an account" (Chrysostom).
"Fearing the future reckoning, the divine Apostle often presses his disciple with such a charge or adjuration. Yet he does this also from love, wishing to make him glorious and circumspect, and to dispose him toward zealous and bold preaching" (Theodoret).
"The living and the dead." The living are those whom the judgment will find alive, "for many will remain alive until the last judgment" (Chrysostom). The dead are those who will have died before that time.
"The Apostle called the Lord Judge of the living and the dead because He will raise the dead and bring them to judgment. He will also demand an account from those whom the end of the world finds alive, first clothing them with incorruption. For it is said: We shall not all sleep, but we shall all be changed (cf. 1 Cor 15:51)" (Theodoret).
"At His appearing and His kingdom." This means at the time when He comes a second time to reveal His glorious kingdom. Or thus: "When will He judge? At the time of His coming in glory, with royal majesty, that is, not as He appears now" (Chrysostom). "For the second time He will come not as He did the first time, that is, not in humiliation" (Theophylact).