Charles H. Spurgeon on Psalms 61:7

O prepare mercy. David having declared in his own behalf the purpose of God towards him for everlasting salvation, he, speaking of himself, shall abide before God for ever: he withal considering what he was to run through in this life, and what it might require to keep him unto the end, and so for ever, doth presently thereupon, in way of prayer, subjoin prepare mercy and truth, which may preserve me. As if he had said, I have yet a long journey to go, and through many hazards, and thy promise is, I shall abide before thee for ever. Lord, thou hast need lay up and aforehand prepare an abundance of mercy and truth to preserve me for time to come. Thomas Goodwin.