I will lift up mine eyes, etc. In thy agony of a troubled conscience always look upwards unto a gracious God to keep thy soul steady; for looking downward on thyself thou shalt find nothing but what will increase thy fear, infinite sins, good deeds few, and imperfect: it is not thy faith, but God's faithfulness thou must rely upon; casting thine eyes downwards on thyself, to behold the great distance betwixt what you deserve and what thou desirest, is enough to make thee giddy, stagger, and reel into despair. Ever therefore lift up thine eyes unto the hills, from whence cometh thy help, never viewing the deep dale of thy own unworthiness, but to abate thy pride when tempted to presumption.—Thomas Fuller (1608-1661), in "The Cause and Cure of a Wounded Conscience."
Quoted in Charles Spurgeon's The Treasury of David, on Psalm 121: 1
Fuller was regarded as a witty writer, and a good selection of pithy quotes are available here.