John Calvin on Romans 13:12

12. The night has advanced, and the day, etc. This is the season which he had just mentioned; for as the faithful are not as yet received into full light, he very fitly compares to the dawn the knowledge of future life, which shines on us through the gospel: for day is not put here, as in other places, for the light of faith, (otherwise he could not have said that it was only approaching, but that it was present, for it now shines as it were in the middle of its progress,) but for that glorious brightness of the celestial life, the beginnings of which are now seen through the gospel.

The sum of what he says is, — that as soon as God begins to call us, we ought to do the same, as when we conclude from the first dawn of the day that the full sun is at hand; we ought to look forward to the coming of Christ.

He says that the night had advanced, because we are not so overwhelmed with thick darkness as the unbelieving are, to whom no spark of life appears; but the hope of resurrection is placed by the gospel before our eyes; yea, the light of faith, by which we discover that the full brightness of celestial glory is nigh at hand, ought to stimulate us, so that we may not grow torpid on the earth. But afterwards, when he bids us to walk in the light, as it were during the day time, he does not continue the same metaphor; for he compares to the day our present state, while Christ shines on us. His purpose was in various ways to exhort us, — at one time to meditate on our future life; at another, to contemplate the present favor of God.