Charity
Jesus makes charity the new commandment. By loving his own "to the end," he makes manifest the Father's love which he receives. By loving one another, the disciples imitate the love of Jesus which they themselves receive. Whence Jesus says: "As the Father has loved me, so have I loved you; abide in my love." and again: "This is my commandment, that you love one another as I have loved you."
Source: Catechism of the Catholic Church (Vatican.va)