"A Call to Prayer and Testimony" was preached by C.H. Spurgeon at the Metropolitan Tabernacle in London on February 8th, 1891. His text that morning was Isaiah 62:6-7: "On your walls, O Jerusalem, I have set watchmen; all the day and all the night they shall never be silent. You who put the LORD in remembrance, take no rest, and give him no rest until he establishes Jerusalem and makes it a praise in the earth". Here from that sermon is Spurgeon's fifth reason not to rest:
Above all, let us never rest out of despair. The feeling does come over you sometimes—"What is the use of our labor? So little comes of it. What is the use of protesting for the truth? The churches will not hear you. You only earn ill-will, and are ridiculed as an old fogy. What is the use of being earnest about winning souls? Men are indifferent. The present engrosses thought—social questions are pressing. Everybody pines for sensationalism or amusement. What profit is there in keeping to the old way?"
That spirit creeps over the child of God like the cold of the Arctic regions, numbing him and tending to send him into the sleep of despair. The evidence of this evil power is found in the tendency to restrain prayer before God. From this may our God rescue us! Come, my brothers, I do not know who among you is going to sleep; but I would like to shake the man who is so benumbed, and wake him up; and I hope that, in your turn, when you see me benumbed, you will shake me also, and wake me up to diligence in prayer. Let us awake this morning, and begin again.
We must not, will not, yield to slumber. There is small cause for fear, and no cause for despair. Our cause defeated? Not a bit of it! All will come right yet. God waits; but he waits that he may be gracious unto us. His time to favor Zion will come, and the good old cause will win the victory. The work of the Lord is in a greater hand than ours. He will not fail nor be discouraged. "Men ought always to pray, and not to faint"; and when they feel that they are fainting, they should resolve to pray with double earnestness, and faintness will yield to joy.
(You can read the whole wonderful thing here.)