John Calvin Commentary Jeremiah 50:31

"Behold, I am against thee, O thou proud one, saith the Lord, Jehovah of hosts; for thy day is come, the time that I will visit thee." — Jeremiah 50:31 (ASV)

Jeremiah, to more fully confirm what he had said, again introduces God as the speaker. And we have stated how necessary this was, because he could hardly have gained credit for his prophecy otherwise; but when he introduced God, he removed every doubt. Behold, he says, I am against thee, O proud one. He again calls the Babylonians proud, even because they had not been led to war by levity, folly, or vain ambition, but because they had assailed God and men without any reverence and without any regard for humanity.

He says that the time had come, because the faithful would have otherwise interrupted him and said, “How is it that God delays so long?” So that they might then sustain and cherish hope until the time God had prescribed for his vengeance, he says that the day had come, and the time of visitation.

Whenever this way of speaking occurs, let us understand that all the natural instincts of our flesh are checked. For every one of us immediately jumps to take vengeance when we see the faithful oppressed, when we see many unworthy things done to our brethren, when we see innocent blood shed, and the miserable cruelly treated by the ungodly. Therefore, when all these instances of barbarity happen, none of us can contain ourselves; hence God puts a bridle on us and exhorts us to exercise patience, when he says that the time of visitation is not yet complete.

So, as long as God delays, let us understand that the fitting time has not yet come, because he has a fixed day of visitation, unknown to us.