This brief excerpt from Pilgrim's Progress provides insight as to why we, as servants of the Lord, are able to persevere even after our human strength has already given out. We must not minimize the power of the Holy Spirit to inspire and renew God's people. The Father has invested the life of His own Son for our deliverance, and is not limited in his ability to continually resource us for His glory.
"Then I saw in my dream, that the Interpreter took Christian by the hand, and led him into a place where was a fire burning against a wall. Standing by the wall was an individual continually throwing water on the fire to put it out. Yet the fire just burned higher and hotter.
Christian asked, “What does this mean?”
The Interpreter answered, “This fire is the work of grace working in the heart. He who throws water on it, to extinguish it and put it out, is the devil. But as you see, the fire is burning higher and hotter in spite of it. You'll be shown the reason of it.”
With that he took Christian to the other side of the wall. There he saw a man with a vessel of oil in his hand, of the which he did also continually cast (but secretly) into the fire.
Then said Christian, “What means this?”
The Interpreter answered, “This is Christ, who continually, with the oil of his grace, maintains the work already begun in the heart; by the means of which, notwithstanding what the devil can do, the souls of his people prove gracious still. And in that thou sawest that the man stood behind the wall to maintain the fire; this is to teach thee, that it is hard for the tempted to see how this work of grace is maintained in the soul.”
And He said to me, “My grace is sufficient for you, for My strength is made perfect in weakness.” Therefore most gladly I will rather boast in my infirmities, that the power of Christ may rest upon me.
2 Corinthians 12:9 (NKJV)