John Calvin Commentary Jeremiah 38:18

"But if thou wilt not go forth to the king of Babylon`s princes, then shall this city be given into the hand of the Chaldeans, and they shall burn it with fire, and thou shalt not escape out of their hand." — Jeremiah 38:18 (ASV)

The Prophet gave the king the hope of pardon; not that he promised impunity, but that the king might at least hope God would be merciful to him, if he anticipated His extreme vengeance. But since hypocrites are not easily moved when God allures them by the sweetness of His promises, a threatening is therefore added, “Except you deliver yourself up,” says the Prophet, “to the Chaldeans, you shall not escape, and the city shall be taken and burnt by the Chaldeans.”

Zedekiah might have had hope in part, and thus have found the mercy God offered him. Since he had profited nothing in this respect, it was necessary, in another way, to arouse him by setting before him the destruction of the city and his own death. But he was not prevailed upon either by fear or by hope to obey the advice of the Prophet. We therefore see that, though he did not openly despise God, he was still neither cold nor hot, but wished to be wholly spared. It was for this reason, then, that he rejected the favor offered to him by the Prophet.