What communion does light have with darkness?
Light has no communion with darkness. They are two distinct opposites that will not cohabit, and you cannot have both. You will have either light or darkness, never a mixture, because righteousness and unrighteousness cannot share fellowship. The two are opposite and cannot be mixed.
Les teaches that the question about light and darkness follows the earlier question, 'What fellowship hath righteousness with unrighteousness?' The answer is that they are two distinct opposites. Light and darkness will not cohabit; you cannot have both at the same time. A person is either in light or in darkness. This same principle of an unequal yoke applies not only to marriage but also to business or any common cause where people must work together. In verse 15, Les notes the question about what concord Christ has with Belial, pointing out that Belial is used only here in the whole New Testament.
Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?