John Calvin Commentary Jeremiah 42:4

"Then Jeremiah the prophet said unto them, I have heard you; behold, I will pray unto Jehovah your God according to your words; and it shall come to pass that whatsoever thing Jehovah shall answer you, I will declare it unto you; I will keep nothing back from you." — Jeremiah 42:4 (ASV)

To prepare them to obey, he testified that he would be a faithful messenger of God, for there is no doubt that the Prophet, as we will see, regarded them with suspicion. Therefore, so that they might be teachable and obedient to the answer expected from God, he stated beforehand that he would honestly and faithfully fulfill his role as a prophet.

I have heard, he says; here he shows how ready he was to listen, and how he neglected nothing contributing to their well-being. I have heard, he says, Behold, I will pray according to your words. There is no doubt that he thus indicated that he wished them well; and knowing that the Prophet was motivated by love might have made them more attentive to the oracle. Nor is there any doubt that the Prophet testified his love towards them, so that his doctrine might subsequently carry more weight with them.

By saying, Whatever your God will answer, he did not mean that the oracle would be revealed to everyone directly, for the words could only be explained through the Prophet, who would openly make known to the whole people what he heard from God’s mouth. But he says that the answer would be given to them, because God would give the answer that was to be communicated to all, just as it is said that God spoke to Moses, and also to all the people, for the doctrine was intended for all. Moses did not receive the law, nor its interpretation, in his private capacity, but so that the people might know what was right. Jeremiah did the same here; the answer he received from God he made known as belonging to all the people collectively.

But in calling God their God, he did not mean to flatter them or to praise their piety. Instead, his purpose was to exhort them to surrender and devote themselves wholly to God. It was as if he had said that they had to do with God, who had bound them to Himself when He adopted them as His own special people and then favored them with so many blessings.

Since God had therefore made Himself known to them, they could not reject His counsel without punishment, for there was no excuse of ignorance. Thus, we see the weight in this phrase, your God. For Jeremiah reminded them that they could not trifle with God without punishment, because they did not belong to themselves but had been chosen as God’s people, on the condition that they be wholly subject to His authority.

So, the sum of it all is this: the Prophet would faithfully convey to the Jews the answer God would give them, and he said this so that his doctrine might have greater authority among them.