Christian morality isn’t just any morality that a Christian might choose, as though the fact that a Christian invokes it might somehow make it worthy of the label. It has at least one distinctive characteristic.
It is Jesus himself who takes the initiative and calls people to follow him. His call is addressed first to those to whom he entrusts a particular mission, beginning with the Twelve; but it is also clear that every believer is called to be a follower of Christ (cf. Acts 6:1). Following Christ is thus the essential and primordial foundation of Christian morality: just as the people of Israel followed God who led them through the desert towards the Promised Land (cf. Ex 13:21), so every disciple must follow Jesus, towards whom he is drawn by the Father himself (cf. Jn 6:44).
This is not a matter only of disposing oneself to hear a teaching and obediently accepting a commandment. More radically, it involves holding fast to the very person of Jesus, partaking of his life and his destiny, sharing in his free and loving obedience to the will of the Father. By responding in faith and following the one who is Incarnate Wisdom, the disciple of Jesus truly becomes a disciple of God (cf. Jn 6:45). Jesus is indeed the light of the world, the light of life (cf. Jn 8:12). He is the shepherd who leads his sheep and feeds them (cf. Jn 10:11-16); he is the way, and the truth, and the life (cf. Jn 14:6). It is Jesus who leads to the Father, so much so that to see him, the Son, is to see the Father (cf. Jn 14:6-10). And thus to imitate the Son, “the image of the invisible God” (Col 1:15), means to imitate the Father.
20. Jesus asks us to follow him and to imitate him along the path of love, a love which gives itself completely to the brethren out of love for God: “This is my commandment, that you love one another as I have loved you” (Jn 15:12). The word “as” requires imitation of Jesus and of his love, of which the washing of feet is a sign: “If I then, your Lord and Teacher, have washed your feet, you also ought to wash one another’s feet. For I have given you an example, that you should do as I have done to you” (Jn 13:14-15). Jesus’ way of acting and his words, his deeds and his precepts constitute the moral rule of Christian life. Indeed, his actions, and in particular his Passion and Death on the Cross, are the living revelation of his love for the Father and for others. This is exactly the love that Jesus wishes to be imitated by all who follow him. It is the “new” commandment: “A new commandment I give to you, that you love one another; even as I have loved you, that you also love one another. By this all men will know that you are my disciples, if you have love for one another” (Jn 13:34-35).
The word “as” also indicates the degree of Jesus’ love, and of the love with which his disciples are called to love one another. After saying: “This is my commandment, that you love one another as I have loved you” (Jn 15:12), Jesus continues with words which indicate the sacrificial gift of his life on the Cross, as the witness to a love “to the end” (Jn 13:1): “Greater love has no man than this, that a man lay down his life for his friends” (Jn 15:13). [Veritatis Splendor §§19-20; italics in original, boldface added]
The principle of charity must inform the Christian man’s actions. He has no freedom to choose another principle. Out of love for Christ, charity towards others must be the foundation of his actions; otherwise, his morality is not Christian.