John Gill 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)

Then Jeremiah the prophet said to them, I have heard [you] , &c.] He took notice of what they said to him, and found himself disposed to comply with their request, and readily granted it: behold, I will pray to the Lord your God, according to your words ;

Be an intercessor for them; use his interest with his God, and their God; and, on account of relation, might expect to be heard; whom he would humbly entreat to direct what they should do, as they desired: and it shall come to pass, [that] whatsoever thing the Lord shall answer you, I will declare [it] to you; I will keep nothing back from you ;

But faithfully make known the whole mind and will of God, just as it is delivered, be it in what way soever: and though it is not expressed, he might suggest that he had some doubt on his mind whether they would obey it or not; and that he expected they would be open and free in declaring themselves on that point; since he had so readily complied with their request, and was determined to act the faithful part to them; hence the following reply: