For great is thy mercy toward me - In respect to me; or, Thou hast manifested great mercy to me; to wit, in past times. He makes use of this now as an argument or reason why God should interpose again.
(a) He had shown on former occasions that he had power to save;
(b) the fact that he had thus treated him as his friend was a reason why he should now befriend him.
And thou hast delivered my soul - My life. The meaning is, that he had kept him alive in times of imminent danger. At the same time David could say, as every child of God can say, that God had delivered his soul in the strict and proper sense of the term - from sin, and death, and hell itself.
From the lowest hell - Margin, grave; Hebrew, שׁאול she'ôl; Greek, ᾅδης Hadēs. See the word explained in the notes at Isaiah 14:9. Compare the notes at Job 10:21-22. The word rendered “lowest” means simply under, or beneath: the grave or hades beneath. The idea of lowest, or the superlative degree, is not necessarily implied in the word. The idea of the grave as deep, or as under us, however, is implied, and the psalmist means to say that he had been saved from that deep dwelling-place - from the abode of departed spirits, to which the dead descend under ground. The meaning is, that he had been kept alive; but the greatness of the mercy is designed to be set forth by having before the mind a vivid idea of the darkness, the horror, and the gloom of the world to which the dead descend, and where they dwell.