John Calvin on Lamentations 4:12

He confirms the same thing; for when a thing incredible happens, either we are extremely stupid, or we must be moved and affected. The Prophet, then, now says that the destruction of the city of Jerusalem had been incredible, because God had defended it by his power; it was also so fortified that no one believed that it could be taken, and the grandeur of the city was known everywhere.

He then says that Jerusalem had been taken and overthrown, which no one of the heathens, neither their kings nor their people, had thought possible. It then follows that the city had been destroyed by God’s hand rather than by the power of enemies. Nebuchadnezzar had indeed brought a strong army, but the city was so well fortified that they thought that all attempts would be in vain. That the city, then, was taken and demolished, could not have been ascribed to human forces, but to a power hidden from the eyes of men. It then follows that it was God’s work, and indeed singular. We now, then, understand the design of the Prophet in saying that it was not believed by kings nor people that enemies could storm Jerusalem. And in continuation he adds, —

Lamentations 4:13

13. For the sins of her prophets, and the iniquities of her priests, that have shed the blood of the just in the midst of her,

13. Propter peccatum prophetarum ejus, iniquitatem sacerdotum ejus, qui fuderunt in medio ejus sanguinem justorum.