I have been going through the three epistles of John, and have been meditating on them lately. The permeating theme in these letters is "love" -- that is, God's definition, and not the world's. Love is the nature of God. All of His justice, mercy, glory -- all of these atributes spring from His perfect love.

Since God is love, we, having been born again to Him are to partake of His nature. God wants us to be perfected in love. Love is the mark of the true believer, for it cannot be counterfeited or faked for long, especially in the hour of trial. God tests the hearts, and He knows how much we truly love Him and love those who have fellowship with Him.

Now by this we know that we know Him, if we keep His commandments. He who says, “I know Him,” and does not keep His commandments, is a liar, and the truth is not in him. But whoever keeps His word, truly the love of God is perfected in him. By this we know that we are in Him. He who says he abides in Him ought himself also to walk just as He walked.
Brethren, I write no new commandment to you, but an old commandment which you have had from the beginning. The old commandment is the word which you heard from the beginning. Again, a new commandment I write to you, which thing 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 darkness until now. He who loves his brother abides in the light, and there is no cause for stumbling in him. But he who hates his brother is in darkness and walks in darkness, and does not know where he is going, because the darkness has blinded his eyes.