Psalm 30:6

And in my prosperity
Either outward prosperity, when he was settled in his kingdom, and as acknowledged king by all the tribes of Israel, and had gotten the victory over all his enemies, and was at rest from them round about; or inward and spiritual prosperity, having a spiritual appetite for the word, being in the lively exercise of grace, growing in it, and in the knowledge of Christ; favoured with communion with God, having flesh discoveries of pardoning grace and mercy, corruptions being subdued, the inward man renewed with spiritual strength, and more fruitful in every good word and work. This being the case,

I said, I shall never be moved;
so in outward prosperity men are apt to sing a requiem to themselves, and fancy it will always be thus with them, be in health of body, and enjoying the affluence of temporal things, and so put away the evil day in one sense and another from them; and even good men themselves are subject to this infirmity, ( Job 29:18-20 ) ; and who also, when in comfortable frames of soul, and in prosperous circumstances in spiritual things, are ready to conclude if will always be thus with them, or better. Indeed they can never be moved as to their state and condition with respect to God; not from his heart, where they are set as a seal; nor out of the arms of Christ, and covenant of grace; nor out of the family of God; nor from a state of justification and grace; but they may be moved as to the exercise of grace and discharge of duty, in which they vary; and especially when they are self-confident, and depend upon their own strength for the performance of these things, and for a continuance in such frames, which seems to have been David's case; and therefore he corrects himself, and his sense of things, in ( Psalms 30:7 ) .