The night is far gone, the day is at hand (Romans (13:12).
The import is that the entire time of the present life is compared to night on account of the darkness of ignorance with which the present life is encumbered.
“We are swallowed up in darkness” (Jb 33:4). Isaiah says of this night: “My soul yearns for thee in the night” (26:9).
But the state of future happiness is compared to day on account of God’s splendor with which the saints are enlightened: “the sun shall be no more your light by day, nor for brightness shall the moon give light to you by night, but he Lord will be your everlasting light” (Is 60:19)….
It can also be understood that the state of guilt is being compared to night on account of the darkness of guilt. About this darkness Ps 82 (v. 5) says: “They have neither knowledge nor understanding; they walk about in darkness”….
But day is called the state of grace on account of the light of spiritual understanding which the just have, but the wicked lack: “Light dawns for the righteous” (Ps 97:11); “The light of justice did not shine on us” (Wis 5:6).
Or it can be understood that the time before Christ’s incarnation is being compared to night, because it was not yet clear but wrapped in darkness….
Hence, just as shadows appear at night, so during that time the practices of the Law were in vogue, but “these were only a shadow of what is to come” (Col 2:17).
But the time after Christ’s incarnation is compared to day on account of the power of the spiritual sun in the world: “But for you who fear my name, the sun of justice shall shine” (Mal 4:2)….
The saying, the night is far gone, can be taken for any of the three nights mentioned….
But it seems that the saying, the day is at hand, must be understood as referring to the day of future glory, which was at hand for the believers in Christ to whom he was writing, although it had not yet arrived for them.
In keeping with the foregoing explanation, the time of Christ’s grace, although it had already arrived as regards the passage of time, is nonetheless described as drawing near through faith and devotion; just as it also says in Phil 4:5 “The Lord is near,” and in Ps 145:18 “The Lord is near to all who call upon him.”
It can also apply to those who begin to repent of their sins; for such persons the day of grace is at hand.
Thomas Aquinas (1225-1274): Commentary on Romans, cap. 13, lect.3, 1067-1069.