John Gill

They speak vanity everyone with his neighbour,.... That which is false and a lie, either doctrinal or practical; what was not according to the word of God, and was vain and empty, frothy, filthy, and corrupt; and which no godly and faithful man would do. And this being done in common, by the generality of men, one with another, shows the degeneracy of the age, and supports the complaint before made. They speak even with flattering lips; as Cain did to Abel, Joab to Amasa, the Herodians to Christ, Judas to his Master, false teachers to those that are simple, hypocrites to God himself, when they draw nigh to him only with their lips, and all formal professors to the churches of Christ, when they profess themselves to be what they are not. And this is a further proof of the justness of the above complaint; and with a double heart do they speak: or "with an heart and an heart" (d); such are double minded men, who say one thing, and mean another; their words are not to be depended upon; there is no faithfulness in them. The Chinese (e) reckon a man of "two hearts", as they call him, a very wicked man, and none more remote from honesty. (d) "in corde & corde", V. L. Pagninus, Montanus, Gejerus. (e) Martin. Sinic. Hist. p. 144. a heart having , a double meaning, as Pittacus says, Laert. in Vit. Pittac. l. 1. p. 53.