I shall not die David speaks like one emerging from the sepulcher. The very same person who says, I shall not die, acknowledges that he was rescued from death, to which he was near as one condemned to it. For a series of years his life was in imminent danger, exposed every moment to a thousand deaths, and no sooner was he delivered from one than he entered into another. Thus he declares that he would not die, because he regained life, all hope of which he had entirely abandoned. We, whose life is hid with Christ in God, ought to mediate upon this song all our days, Colossians 3:3. If we occasionally enjoy some relaxation, we are bound to unite with David in saying, that we who were surrounded with death are risen to newness of life. In the meantime, we must constantly persevere through the midst of darkness: as our safety lies in hope, it is impossible that it can be very visible to us. In the second member of the verse, he points out the proper use of life. God does not prolong the lives of his people, that they may pamper themselves with meat and drink, sleep as much as they please, and enjoy every temporal blessing, but to magnify him for his benefits which he is daily heaping upon them. Of this subject we have spoken onPsalm 115

I shall not die, but live - Evidently the psalmist had apprehended that he would die; or, he had felt that he was in imminent danger of dying. In this language he seems, as in Psalm 118:13, to go back again to the scenes referred to in the psalm. He lives them over again. He describes the feelings which he had then. He saw that he was in danger. His enemies were thick round about him, and sought his life. But he had then the assurance that they would not be victorious; that they would not accomplish their object; that he would be protected; that he would live to declare what God had done for him. He does not say how he had this assurance, but there is no impropriety in supposing that he had it, as Hezekiah had in similar circumstances (see Isaiah 38:5-8, Isaiah 38:21), by a direct divine intimation. Things like this are not uncommon now, when, in danger or in sickness, the mind is strongly impressed with the belief that there will be a restoration to health and safety, and when the mind is made calm and peaceful by that belief - the very calmness of the mind under such a belief contributing not a little to that result. Why should we hesitate to believe that such a faith and hope may come from the Lord? Compare Acts 27:22-25.
And declare the works of the Lord - Declare what he has done.

I shall not die - I was nigh unto death; but I am preserved, - preserved to publish the wondrous works of the Lord.

I shall not die, but live, and (h) declare the works of the LORD.
(h) So that all, that are both far and near, may see his mighty power.

I shall not die, but live,.... Not that he should never die, David knew he should; but that his present afflictions would not issue in death; or he should not die by the hands of his enemies, he sometimes feared he should; but now believed he should live, as he did, to a good old age: he knew he should live spiritually and eternally, and not die a second death; and so may all true believers and members of Christ say. Yea, these words may be considered as the words of Christ; who, though he came into the world to die, and did die for the sins of his people; yet he knew he should not die before his time, nor should he continue long under the power of death; but should live again, and live for evermore, and not die; death should have no more dominion over him; see Psalm 16:10;
and declare the works of the Lord; the wonderful appearances of God in a providential way, and all his marvellous works of grace; as David did, and as all the people of God more or less do; and which is the end of their living; not to eat and drink, and gratify their carnal senses, but to glorify God, by declaring what he has done for themselves and others. So the Messiah declared the name of God, his nature, perfections, mind and will, word and works, among his brethren in the great congregation, Psalm 22:22.

He would live, because confident his life would be for God's glory.