John Calvin Commentary Psalms 78:10

"They kept not the covenant of God, And refused to walk in his law;" — Psalms 78:10 (ASV)

They kept not the covenant of God. This is the reason assigned for the Ephraimites turning their backs in the day of battle, and it explains why divine assistance was withheld from them. Others, it is true, were guilty in this respect as well as they; but the vengeance of God executed on that tribe, which by its influence had corrupted almost the whole kingdom, is purposely brought forward as a general warning.

Since then the tribe of Ephraim, as a result of its splendor and dignity, when it threw off the yoke, encouraged and became, so to speak, a standard of shameful revolt to all the other tribes, the prophet intended to put people on their guard, so that they might not, in their simplicity, allow themselves to be deceived again in the same way.

It is no light charge that he brings against the sons of Ephraim: he upbraids them for their perfidiousness in despising the whole law and in violating the covenant. Although he uses these two words, law and covenant, in the same sense, yet, by placing the covenant first, he clearly shows that he is speaking not only of the moral law (the all-perfect rule of life) but also of the whole service of God, of the truth and faithfulness of the divine promises, of the trust that should be placed in them, of invocation, and of the doctrine of true religion, the foundation of which was the adoption.

He therefore calls them covenant-breakers, because they had fallen from their trust in the promises by which God had entered into covenant with them to be their Father. Yet he afterwards very properly adds the law, in which the covenant was sealed up, so to speak, in public records. He aggravates the enormity of their guilt by using the word refuse, which intimates that they were not simply carried away by thoughtless or inconsiderate recklessness, sinning through flightiness, lack of knowledge, or foresight, but that they had purposely, and with deliberate obstinacy, violated the holy covenant of God.