Love one another
God first showed his love for mankind by sending his only son to come and die for the sins of man so that men will no more die but live and have eternal life. This show of love is the greatest love ever to be witnessed in history and in time. God did this to demonstrate to us what love is and how he expects us to love one another as we journey on this earth.
The state of the earth currently is full of sin and evil deeds so much that some people even think that there is no God (though they are fools as scripture describes them). The earth is lacking love, which is the medicine that will heal the earth. Jesus admonished that as his followers we should demonstrate and exhibit the love of God amongst ourselves as his disciples. That is the only way that we can prove to men that we are his disciples indeed. Our actions must show and tell of the love of God. That is the only distinguishing feature between us and the people of the world.