Then Jeremiah said, I have heard you, &c. That is, I will do for you what you desire. I will pray unto the Lord your God They called the Lord Jeremiah's God: here Jeremiah calls him their God, both to remind them of God's relation to them, and of their duty toward him. And whatsoever thing the Lord shall answer, I will declare I will be faithful in giving you an account of what God shall reveal to me to be his will concerning you. Then they said, The Lord be a true and faithful witness, &c. The words of this and the following verse imply a perfect oath, the form of which lies in appealing to God as a witness of the sincerity of the hearts of those that swear, for a security to those to whom the oath is given: which also includes a secret challenging of God to take vengeance upon those that give that security, if they should not act according to their promise. The thing these men promise is, that they would perfectly obey God's will, whether agreeable or disagreeable to them. And they further declare they were convinced that their prosperity and happiness entirely depended upon their complying with God's will, adding, That it may be well with us, &c.
Related Commentaries of Jeremiah 42:4
Jeremiah 42:4-6
4 Then Jeremiah the prophet said unto them, I have heard you; behold, I will pray unto the LORD your God according to your words; and it shall come to pass, that whatsoever thing the LORD shall answer you, I will declare it unto you; I will keep nothing back from you.
5 Then they said to Jeremiah, The LORD be a true and faithful witness between us, if we do not even according to all things for the which the LORD thy God shall send thee to us.
6 Whether it be good, or whether it be evil, we will obey the voice of the LORD our God, to whom we send thee; that it may be well with us, when we obey the voice of the LORD our God.