The true Christian is called to be a soldier, and must behave as such from the day of his conversion to the day of his death. He is not meant to live a life of religious ease, indolence, and security. He must never imagine for a moment that he can sleep and doze along the way to heaven, like one travelling in an easy carriage. If he takes his standard of Christianity from the children of this world, he may be content with such notions; but he will find no countenance for them in the Word of God. If the Bible is the rule of his faith and practice, he will find his course laid down very plainly in this matter. He must “fight.”
With whom is the Christian soldier meant to fight? … The principal fight of the Christian is with the world, the flesh, and the devil. These are his never-dying foes. These are the three chief enemies against whom he must wage war. Unless he gets the victory over these three, all other victories are useless and vain. If he had a nature like an angel, and were not a fallen creature, the warfare would not be so essential. But with a corrupt heart, a busy devil, and an ensnaring world, he must either “fight” or be lost.
He must fight the flesh. Even after conversion he carries within him a nature prone to evil, and a heart weak and unstable as water. That heart will never be free from imperfection in this world, and it is a miserable delusion to expect it. To keep that heart from going astray, the Lord Jesus bids us “watch and pray.” The spirit may be ready, but the flesh is weak. There is need of a daily struggle and a daily wrestling in prayer. “I keep under my body,” cries St. Paul, “and bring it into subjection.”—“I see a law in my members warring against the law of mind, and bringing me into captivity.”—“O wretched man that I am, who shall deliver me from the body of this death?”—“They that are Christ’s have crucified the flesh with the affections and lusts.”—“Mortify your members which are upon the earth.” (Mark 14:38; 1 Corinthians 9:27; Romans 7:23, 24; Galatians 5:24; Colossians 3:5)
—J. C. Ryle
Holiness (London: William Hunt and Company, 1889), 76–77.