2 That, then, was called the day of retribution on which the Lord will render to every one according to his works— that is, the judgment. The acceptable year of the Lord, again, is this present time, in which those who believe Him are called by Him, and become acceptable to God— that is, the whole time from His advent onwards to the consummation [of all things], during which He acquires to Himself as fruits [of the scheme of mercy] those who are saved. For, according to the phraseology of the prophet, the day of retribution follows the [acceptable] year; and the prophet will be proved guilty of falsehood if the Lord preached only for a year, and if he speaks of it.
For where is the day of retribution? For the year has passed, and the day of retribution has not yet come; but He still “makes His sun to rise upon the good and upon the evil, and sends rain upon the just and unjust.” And the righteous suffer persecution, are afflicted, and are slain, while sinners are possessed of abundance, and “drink with the sound of the harp and psaltery, but do not regard the works of the Lord.” But, according to the language [used by the prophet], they ought to be combined, and the day of retribution to follow the [acceptable] year.
For the words are, “to proclaim the acceptable year of the Lord, and the day of retribution.” This present time, therefore, in which men are called and saved by the Lord, is properly understood to be denoted by “the acceptable year of the Lord;” and there follows on this “the day of retribution,” that is, the judgment. And the time thus referred to is not called “a year” only, but is also named “a day” both by the prophet and by Paul, of whom the apostle, calling to mind the Scripture, says in the Epistle addressed to the Romans, “As it is written, for your sake we are killed all the day long, we are counted as sheep for the slaughter.” But here the expression “all the day long” is put for all this time during which we suffer persecution, and are killed as sheep.
As then this day does not signify one which consists of twelve hours, but the whole time during which believers in Christ suffer and are put to death for His sake, so also the year there mentioned does not denote one which consists of twelve months, but the whole time of faith during which men hear and believe the preaching of the Gospel, and those become acceptable to God who unite themselves to Him.
Source: Adversus haereses (New Advent)