The psalmist addresses the people of Israel at an annual festival celebrating their deliverance from slavery in Egypt. He begins with a call to joyful celebration (st. 1) and praise of the LORD (st. 2), remembering God's saving answer to the people's cry in bondage and God's test of their faith in the wilderness (st. 3). As
God's spokesperson, the psalmist calls Israel once again to be faithful to the LORD and to reject all false gods (st. 4). In remembering the Exodus, Israel is reminded of its own unfaithfulness in the wilderness and how God dealt with them. The LORD, who brought them out of Egypt and who has supplied all their needs, will surely bless the people if they wholly trust in him (st. 5). God's great desire is for the people to obey and trust him, so that they may enjoy his protection and abundant blessings (st. 6-7). In singing this psalm we too need to respond obediently to God's call. In the post-exilic temple liturgy this psalm was sung during the morning sacrifice on the fifth day of the week. Marie J. Post versified Psalm 81 in 1984 for the Psalter Hymnal.
Psalms for All Seasons added the suggestion to read the exhortation of the Lord from Ps. 81:6-16 or a reading of the Law, followed by st. 3 written by Martin Tel.
Bert Polman, Psalter Hymnal Handbook
It is vitally important that worshipers understand the role of God’s law among us. God gives his law to us, not so that we can earn his favor by full obedience, for even those converted to God cannot obey this law perfectly. Heidelberg Catechism, Lord’s Day 44, Question and Answer 114 says, “In this life even the holiest have only a small beginning of this obedience.” Instead, says Heidelberg Catechism, Lord’s Day 2, Question and Answer 3, through this law “we come to know [our] misery.”
Yet in their new life of gratitude, God’s children “with all seriousness of purpose, do begin to live according to all, not only some, of God’s commandments” (Heidelberg Catechism, Lord’s Day 44, Question and Answer 114). They measure their good works of gratitude as “those which are done out of true faith, conform to God’s law, and are done for God’s glory” (Heidelberg Catechism, Lord’s Day 33, Question and Answer 91).
In other words, though Christ has fulfilled the law for us, “The truth and substance of these things remain for us in Jesus Christ…[and] we continue to use the witnesses drawn from the law and prophets to confirm us in the gospel and to regulate our lives with full integrity for the glory of God according to the will of God” (Belgic Confession, Article 25). Therefore, the Ten Commandments with explanation are included in the third section, “gratitude,” (Lord’s Days 34-44) of Heidelberg Catechism.