The first line in the reading from John's letter is "Let us love one another." The last line in the gospel is the same. There is no doubt that this is the basic thrust of John's message and in between among other things he tells us "how to love" and even more to the point he tells us "what" love is.
But these clarifications do not seem to be all that helpful ...in fact for many of us, the whole concept is much too vague to be really significant.
We are told that we are to love each other as the Father has loved Christ ...but what does that mean? And even more abstract ..."God is love" ...not God is loving or loveable, but God IS love!
Is this a definition of God or of love?
Well, it is neither. It is not a definition at all but rather it is an attempt to express, not to explain but to express the divine reality in the simplest terms.
For if we are to speak of God at all, and we must, we have no choice but to do so in a manner compatible with our understanding and experience.
The most God-like word selected by both John and Paul after much inspired reflection was "love" It could have been "power" or "justice" or "unity" …but it was "love."
In Romans 5, Paul says, "God’s love has been poured out into our hearts through the Holy Spirit which has been given to us."
Thus it is God, who, in the person of the Holy Spirit, enters into us and that divine presence within us manifests itself through our acts of love; or, put in another way, through our sharing God with others.
And so it does make sense to say that 'love' is the very essence of God ...in other words, that God is love.
Which brings us back to Jesus in whom all of this makes more sense than ever.