"O Come, My Soul" is a paraphrase of Psalm 103. The lengthy meter (11 10 11 10) makes possible a clear flow of thoughts and images, and the refrain continually reminds us of the praise running through this much-loved psalm. The versification comes from the 1912 Psalter with minor alterations.
Bert Polman, Psalter Hymnal Handbook
The Catechism says that those who know Christ’s forgiveness are “to thank God for such deliverance” (Heidelberg Catechism, Lord’s Day 1, Question and Answer 2). As a result, “With our whole lives we may show that we are thankful to God for his benefits, so that he may be praised through us, and that we may be assured of our faith by its fruits, and so that by our godly living our neighbors may be won over to Christ” (Heidelberg Catechism, Lord’s Day 32, Question and Answer 86).