I have many a time had doubts and fears, as most of you have had – and where is the strong believer that has not sometimes wavered? I have said, within myself "Is this religion true, which, day after day, I incessantly preach to the people? Is it the correct one? Is it true that this religion has an influence upon mankind?" And I will tell you how I have reassured myself. I have looked upon the hundreds, nay, upon the thousands whom I have around me, who were once the vilest of the vile – drunkards, swearers and such like – and I now see them clothed and in their right mind, walking in holiness and in the fear of God; and I have said, within myself, "This must be the truth, then, because I see its marvellous effects. It is true because it is efficient for purposes which error never could accomplish. It exerts an influence among the lowest order of mortals, and over the most abominable of our race."
C. H. Spurgeon