There are three chief ways in which we can speak of light. There is physical light, which is the opposite of darkness. There is mental light, that is, knowledge. We sometimes say, "Now I see the light." And there is that important spiritual light which means knowledge of things spiritual.
When then in Ephesians 5:8 Paul writes, "For ye were sometimes darkness, but now are ye children of light," he means and is speaking here of spiritual light.
Satan came with spiritual darkness when he brought the lie to Adam and Eve. They lost their intellectual light as well as their spiritual light; for they did not know God as He is, but saw Him as Satan in his lie had presented Him. In fact he presented God as a liar who told Adam the untruth about that tree of knowledge of good and evil. Through Adam we all lost our spiritual light, and we hold on to the lie.
For those who are in Christ a change has come. His cross has earned for them the right to be spiritually enlightened, and to know God as God, and not as Satan presented Him. They now know Christ as the light of the world. He says that Himself in John 8:12 . What He means is that He is the spiritual light of the world. And in I Peter 2:9 we have that comforting truth that now, through Christ, we are called, that is, brought out of darkness into the marvelous spiritual and intellectual light.
Our calling is then to walk in that light, that is, to walk in the truth, which in turn means to walk in Christ, Who is the Light of the World. Our calling is to walk as Christians. Christians are children of light, children who are spiritually like Christ. That we are like Christ must be seen in all we say and do, in what we love and what we hate, and in that we believe what God says and rejects Satan and all his lies.
Read: Ephesians 1:21 .
“We hold it, therefore, as indubitable, indeed it should be notorious to all tolerably versant with Scripture, that the most splendid works performed by men, who are not yet truly sanctified, are so far from being righteousness in the sight of the Lord, that he regards them as sins…The most perfect thing which proceeds from man is always polluted by some stain….For a long time the world has been taught differently. A kind of good works called moral has been found out, by which men are rendered agreeable to God before they are ingrafted into Christ; as if Scripture spoke falsely when it says, “He that hath the Son hath life, and he that hath not the Son of God hath not life” ( I John 5:12). How can they produce the materials of life if they are dead? Is there no meaning in its being said, that “whatsoever is not of faith is sin”? (Romans 14:23) –John Calvin