And God separated the light from the darkness.Genesis 1:4
A believer has two principles at work within him. In his natural estate he was subject to one principle only, which was darkness; now light has entered, and the two principles disagree. Consider the apostle Paul's words in the seventh chapter of Romans: "I find it to be a law that when I want to do right, evil lies close at hand. For I delight in the law of God, in my inner being, but I see in my members another law waging war against the law of my mind and making me captive to the law of sin that dwells in my members."1 How is this state of things occasioned? "God separated the light from the darkness." Darkness, by itself, is quiet and undisturbed, but when the Lord sends in light, there is a conflict, for the one is in opposition to the other, a conflict that will never end until the believer is altogether light in the Lord.
If there is a division inside the individual Christian, there is certain to be a division outside. As soon as the Lord gives light to any man, he proceeds to separate himself from the darkness around; he withdraws from a merely worldly religion of outward ceremony, for nothing short of the Gospel of Christ will now satisfy him, and he removes himself from worldly society and frivolous amusements and seeks the company of the saints, for "We know that we have passed out of death into life, because we love the brothers."2
The light gathers to itself, and the darkness to itself. What God has separated, let us never try to unite; but as Christ went outside the camp, bearing His reproach, let us come out from the ungodly and be a special people. He was holy, harmless, undefiled, separate from sinners; and as He was, so we are to be nonconformists to the world, dissenting from all sin, and distinguished from the rest of mankind by our likeness to our Master.
1) Romans 7:21-23
2) 1 John 3:14