John Calvin Commentary Psalms 78:36

"But they flattered him with their mouth, And lied unto him with their tongue." — Psalms 78:36 (ASV)

And they flattered him with their mouth, and lied to him with their tongue. Here they are charged with treachery, because they neither confessed their guilt with sincerity of heart, nor truly ascribed to God the glory of their deliverance. We are not to suppose that they made no acknowledgment at all; but it is implied that the confession of the mouth, as it did not proceed from the heart, was forced and not voluntary.

This is well worth noticing; for from it we learn not only our duty to guard against that gross hypocrisy which consists in saying with the tongue before people one thing, while we think a different thing in our hearts, but also that we should beware of a type of hypocrisy which is more hidden. This hidden hypocrisy consists in the sinner, being forced by fear, flattering God in a servile manner, while yet, if he could, he would avoid the judgment of God.

Most people are deeply afflicted with this disease; for although the divine majesty compels from them some kind of awe, yet they would be pleased if the light of divine truth were completely extinguished. It is, therefore, not enough to give assent to the divine word, unless that assent is accompanied with true and pure affection, so that our hearts are not double or divided.

The Psalmist points out the cause and source of this pretense to be that they were not steadfast and faithful. By this he implies that whatever does not proceed from genuine purity of heart is considered lying and deceit in the sight of God. Since this uprightness is everywhere required in the law, he accuses the people of being covenant-breakers, because they had not kept the covenant of God with that faithfulness which was fitting for them. As I have observed elsewhere, we must always presuppose a mutual relationship and correspondence between the covenant of God and our faith, so that the genuine consent of the latter may correspond to the faithfulness of the former.