Charles Spurgeon explains that, just as a mother cannot kill her child (assuming, for the sake of argument, a virtuous mother), so a sinner cannot come to Christ unless the Spirit draws him. Listen to what he writes:

“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.”

He then illustrates what he means:

“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.”