I do not know what you may think, but I can say that I cannot pray but I sin—I cannot preach to you or any others but I sin—I can do nothing without sin; and, as one expresseth it, my repentance wants to be repented of, and my tears to be washed in the precious blood of my dear Redeemer. Our best duties are as so many splendid sins. Before you can speak peace in your heart, you must not only be made sick of your original and actual sin, but you must be made sick of your righteousness, of all your duties and performances. There must be a deep conviction before you can be brought out of your self-righteousness; it is the last idol taken out of our heart. The pride of our heart will not let us submit to the righteousness of Jesus Christ.
Quote is from Whitefield’s Sermon “The Method of Grace”.
Whitefield was an open air preacher that traveled through England, Scotland, and Colonial America. He was used by God to spark and flourish the Great Awakening along with Jonathan Edwards, Howell Harris, and others during c. 1730-1750. The 2 volume (vol. 1 here and vol. 2 here) or single volume biography written by Arnold Dallimore could not be recommended enough. As well, you can also listen to John Piper’s biographical sermon (or download the video here) on George Whitefield.