John Calvin Commentary 1 John 1:5

"And this is the message which we have heard from him and announce unto you, that God is light, and in him is no darkness at all." — 1 John 1:5 (ASV)

This then is the message, or promise. I do not disapprove of the rendering of the old interpreter, “This is the annunciation,” or message; for though ἐπαγγελία means for the most part a promise, yet, as John speaks here generally of the testimony previously mentioned, the context seems to require the other meaning, unless you were to give this explanation, “The promise which we bring to you includes this, or has this condition attached to it.” Thus, the meaning of the Apostle would become evident to us.

For his purpose here was not to include the whole doctrine of the Gospel, but to show that if we desire to enjoy Christ and his blessings, we are required to be conformed to God in righteousness and holiness. Paul says the same thing in the Epistle to Titus, chapter 2, Appeared has the saving grace of God to all, that denying ungodliness and worldly lusts, we may live soberly and righteously and holily in this world (Titus 2:11–12); except that here he says metaphorically, that we are to walk in the light, because God is light.

But he calls God light, and says that he is in the light; such expressions are not to be taken too strictly. Why Satan is called the prince of darkness is sufficiently evident. When, therefore, God on the other hand is called the Father of light, and also light, we first understand that there is nothing in him but what is bright, pure, and unalloyed; and, secondly, that he makes all things so manifest by his brightness, that he allows nothing vicious or perverted, no spots or filth, no hypocrisy or fraud, to lie hidden. Then the sum of what is said is that since there is no union between light and darkness, there is a separation between us and God as long as we walk in darkness; and that the fellowship which he mentions cannot exist unless we also become pure and holy.

In him is no darkness at all. This mode of speaking is commonly used by John to amplify what he has affirmed by a contrary negation. Then, the meaning is that God is such a light that no darkness belongs to him. It therefore follows that he hates an evil conscience, pollution, and wickedness, and everything that pertains to darkness.