John Gill Commentary Jeremiah 5:2

"And though they say, As Jehovah liveth; surely they swear falsely." — Jeremiah 5:2 (ASV)

And though they say, the Lord lives
It might be said, that there were multitudes that made mention of the name of the Lord, that professed it, and swore by it; which sometimes is put for the worship and service of God, (Deuteronomy 10:20) and therefore it could not be so difficult a matter to find a man of integrity and uprightness among them; this is answered by allowing there were persons that did do so: but then it must be observed, that surely they swear falsely ;
they abused the name of God, and were guilty of perjury: or the sense is, they were only nominal professors, hypocritical worshippers; in words professed to know God, but in works denied him; had a form of religion and godliness, but without the power of it.