Permit me to show you wherein this inability of man really does lie. It lies deep in his nature. Through the fall, and through our own sin, the nature of man has become so debased, and depraved, and corrupt, that it is impossible for him to come to Christ without the assistance of God the Holy Spirit. Now, in trying to exhibit how the nature of man thus renders him unable to come to Christ, you must allow me just to take this figure. You see a sheep; how willingly it feeds upon the herbage! You never knew a sheep sigh after carrion; it could not live on lion’s food. Now bring me a wolf; and you ask me whether a wolf cannot eat grass, whether it cannot be just as docile and as domesticated as the sheep. I answer, no; because its nature is contrary thereunto. You say, “Well, it has ears and legs; can it not hear the shepherd’s voice, and follow him whithersoever he leadeth it?” I answer, certainly; there is no physical cause why it cannot do so, but its nature forbids, and therefore I say it cannot do so. Can it not be tamed? cannot its ferocity be removed? Probably it may so far be subdued that it may become apparently tame; but there will always be a marked distinction between it and the sheep, because there is a distinction in nature. Now, the reason why man cannot come to Christ, is not because he cannot come, so far as his body or his mere power of mind is concerned, but because his nature is so corrupt that he has neither the will nor the power to come to Christ unless drawn by the Spirit. But let me give you a better illustration. You see a mother with her babe in her arms. You put a knife into her hand, and tell her to stab that babe to the heart. She replies, and very truthfully, “I cannot.” Now, so far as her bodily power is concerned, she can, if she pleases; there is the knife, and there is the child. The child cannot resist, and she has quite sufficient strength in her hand immediately to stab it to its heart. But she is quite correct when she says she cannot do it. As a mere act of the mind, it is quite possible she might think of such a thing as killing the child, and yet she says she cannot think of such a thing; and she does not say falsely, for her nature as a mother forbids her doing a thing from which her soul revolts. Simply because she is that child’s parent she feels she cannot kill it. It is even so with a sinner. Coming to Christ is so obnoxious to human nature that, although, so far as physical and mental forces are concerned, (and these have but a very narrow sphere in salvation) men could come if they would: it is strictly correct to say that they cannot and will not unless the Father who hath sent Christ doth draw them.
Source: Sermon 182 – C.H. Spurgeon