John Gill Commentary Psalms 78:37

"For their heart was not right with him, Neither were they faithful in his covenant." — Psalms 78:37 (ASV)

For their heart was not right with him
Neither prepared and ready to any good work, but reprobate thereunto; nor steady, fixed, and established, as a good man's heart is, trusting in the Lord; but wavering, fickle, and inconstant; nor true, faithful, and upright; but turning aside like a deceitful bow, as is afterwards said, (Psalms 78:57) ,

neither were they steadfast in his covenant ;
which was made with them at Sinai, though they promised to be obedient, and to do all the Lord said unto them; but this covenant they broke, though he were an husband to them; see (Exodus 24:7) (Jeremiah 31:32) .