They speak vanity - This is a statement of the “manner” in which the “godly” and the “faithful” fail, as stated in Psalms 12:1. One of the ways was that there was a disregard of truth; that no confidence could be placed on the statements of those who professed to be pious; that they dealt falsely with their neighbors. The word “vanity” here is equivalent to “falsehood.” What they spoke was a vain and empty thing, instead of being the truth. It had no reality, and could not be depended on.
Every one with his neighbour - In his statements and promises. No reliance could be placed on his word.
With flattering lips - Hebrew, “Lips of smoothness.” The verb from which the word used here is derived - חלק chālaq - means properly to divide, to distribute; then, to make things equal or smooth; then, to make smooth or to shape, as an artisan does, as with a plane; and then, “to make things smooth with the tongue,” that is, “to flatter.” See Psalms 5:9; Proverbs 5:3; Proverbs 26:28; Proverbs 28:23; Proverbs 29:5. The meaning is, that no confidence could be placed in the statements made. There was no certainty that they were founded on truth; none that they were not intended to deceive. Flattery is the ascribing of qualities to another which he is known not to possess - usually with some sinister or base design.
And with a double heart - Margin, as in Hebrew, “a heart and a heart;” that is, as it were, with two hearts, one that gives utterance to the words, and the other that retains a different sentiment. Thus, in Deuteronomy 25:13, the phrase in Hebrew, “a stone and a stone” means, as it is translated, “divers weights” - one stone or weight to buy with, and another to sell with. So the flatterer. He has one heart to give utterance to the words which he uses toward his neighbor, and another that conceals his real purpose or design. No confidence, therefore, could be placed in such persons. Compare the note at Job 32:22.