I venture to believe, contrary to the opinion of most commentators, that the testimony in Jacob, and the law in Israel here spoken of, had a reference to a much higher subject than the law on mount Sinai. Surely that testimony and that law was Christ himself, who is both the word and the testimony, and the end of the law for righteousness to everyone that believeth. And seen in this point of view, it is most blessed indeed to behold Asaph singing this gospel Psalm with an eye to Christ. Here the latter generations indeed were most highly interested, and the children which were yet unborn. And, as the apostle very sweetly saith, the gospel which was preached to Abraham, and the covenant which was confirmed before of God in Christ, the law, which was 430 years after, cannot disannul. Hence, therefore, this was the grand subject, and this the glorious theme, that the children which should be born should arise and declare. Galatians 3:16-17. To this one point, therefore, ministered all the ordinances, types, sacrifices, and offerings in the old church; and every observance of them pointed to Jesus: they were a shadow of good things to come, but the body was, and is, Christ. Colossians 2:17.
Related Commentaries of Psalms 78:5
Psalms 78:5-8
5 For he established a testimony in Jacob, and appointed a law in Israel, which he commanded our fathers, that they should make them known to their children:
6 That the generation to come might know them, even the children which should be born; who should arise and declare them to their children:
7 That they might set their hope in God, and not forget the works of God, but keep his commandments:
8 And might not be as their fathers, a stubborn and rebellious generation; a generation that set not their heart aright, and whose spirit was not stedfast with God.