And by this we may be sure that we know him, if we keep his commandments. He who says “I know him” but disobeys his commandments is a liar, and the truth is not in him; but whoever keeps his word, in him truly love for God is perfected. By this we may be sure that we are in him: he who says he abides in him ought to walk in the same way in which he walked.
Beloved, I am writing you no new commandment, but an old commandment which you had from the beginning; the old commandment is the word which you have heard. Yet I am writing you a new commandment, which is true in him and in you, because the darkness is passing away and the true light is already shining. He who says he is in the light and hates his brother is in the darkness still. He who loves his brother abides in the light, and in it there is no cause for stumbling. But he who 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.
–1 John 2:3-11