From Oswald Chamber's book, My Utmost for His Highest:
Life without war is impossible either in nature or in grace. The basis of physical, mental, moral, and spiritual life is antagonism. This is the open fact of life.
Health is the balance between physical life and external nature, and it is maintained only by sufficient vitality on the inside against things on the outside. Everything outside my physical life is designed to put me to death. Things which keep me going when I am alive, disintegrate me when I am dead. If I have enough fighting power, I produce the balance of health. The same is true of the mental life. If I want to maintain a vigorous mental life, I have to fight, and in that way the mental balance called 'thought' is produced.
Morally it is the same. Everything that does not partake of the nature of virtue is the enemy of virtue in me, and whether I overcome [them] and produce virtue depends on what moral caliber I have. Immediately I fight, I am moral in that particular. No man is virtuous because he cannot help it; virtue is acquired.
It is the same spiritually as well. Jesus said, "In the world ye shall have tribulation," i.e., everything that is not spiritual makes for my undoing, but "be of good cheer, I have overcome the world." I have to learn to score off the things that come against me, and in that way produce the balance of holiness; then it becomes a delight to meet opposition.