7 Beloved, I am writing you no new commandment, but an old commandment that you had from the beginning. The old commandment is the word that you have heard. 8 At the same time, it is a new commandment that I am writing to you, which is true in him and in you, because the darkness is passing away and the true light is already shining. 9 Whoever says he is in the light and hates his brother is still in darkness. 10 Whoever loves his brother abides in the light, and in him there is no cause for stumbling. 11 But whoever hates his brother is in the darkness and walks in the darkness, and does not know where he is going, because the darkness has blinded his eyes.
There are a number of ideas here that deserve our attention, such as the continuity of Old and New Testaments, or the newness that comes with the Advent of Christ. Also familiar are the contrasts that John uses through his letters, and his gospel, too. What stands out here, though, is the complete incompatibility of the contrasts he presents.
Light and hate are incompatible. If someone claims to have the light of insight or understanding, but has hate in his heart, he is wrong. Love and darkness are likewise incompatible. If a person has love in their heart, they have light in their life.