Saving Faith | An Illustration by Spurgeon

I bet when Spurgeon preached this, the people sat on the edge of their seats (with even a few laughs throughout)...

Faith is something like this. There is a story told of a captain of a man-of-war, whose son—a young lad—was very fond of running up the rigging of the ship; and one time, running after a monkey, he ran up the mast, till at last he got on to the maintruck. Now, the maintruck, you are aware, is like a large round table put on to the mast, so that when the boy was on the maintruck there was plenty of room for him; but the difficulty was—to use the best explanation I can—that he could not reach the mast that was under the table; he was not tall enough to get down from this maintruck, reach the mast, and so descend. There he was on the maintruck; he managed to get up there, somehow or other, but down he never could get. His father saw that, and he looked up in horror; what was he to do? In a few moments his son would fall down, and be dashed to pieces! 

He was clinging to the main-truck with all his might, but in a little time he would fall down on the deck, and there he would be a mangled corpse. The captain called for a speaking trumpet; he put it to his mouth, and shouted, "Boy, the next time the ship lurches, throw yourself into the sea." It was, in truth, his only way of escape; he might be picked up out of the sea, but he could not be rescued if he fell on the deck. The poor boy looked down on the sea; it was a long way; he could not bear the idea of throwing himself into the roaring current beneath him; he thought it looked angry and dangerous. How could he cast himself down into it? So he clung to the main-truck with all his might, though there was no doubt that he must soon let go and perish. The father called for a gun, and pointing it up at him, said, "Boy, the next time the ship lurches, throw yourself into the sea, or I'll shoot you!" He knew his father would keep his word; the ship lurched on one side, over went the boy splash into the sea, and out went brawny arms after him; the sailors rescued him, and brought him on deck. Now, we, like the boy, are in a position of extra-ordinary danger, by nature, which neither you nor I can possibly escape of ourselves. 

Unfortunately, we have got some good works of our own, like that maintruck, and we cling to them so fondly, that we never will give them up. Christ knows that unless we do give them up, we shall be dashed to pieces at the last, for that rotten trust must ruin us. He, therefore, says, "Sinner, let go thine own trust, and drop into the sea of my love." We look down, and say, "Can I be saved by trusting in God? He looks as if he were angry with me, and I could not trust him." Ah, will not mercy's tender cry persuade you?—"He that believeth shall be saved." Must the weapon of destruction be pointed directly at you? Must you hear the dreadful threat—"He that believeth not shall be damned?" It is with you now as with that boy—your position is one of imminent peril in itself, and your slighting the Father's counsel is a matter of more terrible alarm, it makes peril more perilous. 

You must do it, or else you perish! Let go your hold! That is faith when the poor sinner lets go his hold, drops down, and so is saved; and the very thing which looks as if it would destroy him, is the means of his being saved. Oh! believe on Christ, poor sinners; believe on Christ. Ye who know your guilt and misery come, cast yourselves upon him; come, and trust my Master, and as he lives, before whom I stand, you shall never trust him in vain; but you shall find yourselves forgiven, and go your way rejoicing in Christ Jesus.

Source, Spurgeon's Sermon on Justification by Faith