This sentence at the end of what is usually called the parable of the prodigal son wants to show us the magnitude of God's mercy. It closes an entire chapter of Luke's Gospel in which Jesus recounts two parables illustrating the same reality.
Do you remember the episode of the lost sheep and how the owner went after it, leaving the other ninety-nine in the desert (cf. Lk. 15:4-7)? And do you remember the story of the last drachma (Greek silver coin) and the joy of the woman who, having found it, calls her friends and neighbors to rejoice with her (cf. Lk. 15:8-10)?

«Now we must celebrate and rejoice, because your brother was dead and has come to life again; he was lost and has been found.»

With these words, God is extending to you and to all an invitation to rejoice with him, to celebrate and share his joy for the return of the sinner who was lost and has been found.
In the parable, the father addresses these words to the elder son who has always shared in his life, but who after a day of hard work, refuses to enter the house where celebrations have already begun for the return of his brother.
The father goes out to meet the faithful son, just as he had gone out to meet the lost son, and he tries to convince him.
But there is a stark contrast between the sentiments of the father and those of the elder son: the father, with his unlimited love and great joy, which he would like everyone to share in; the son, filled with contempt and jealousy towards his brother whom he no longer recognizes as such. In fact, in referring to his brother, he says: “Your son who has devoured your wealth” (Lk. 15:30).
The father's love and joy for the son who has returned put into even greater contrast the bitterness of the other son, bitterness which reveals a cold and, we could add, false relationship with his father. This elder son is interested in working, in carrying out his duties, but he doesn't love his father as a son should. Rather, we could say that he is obedient to him as one is to a master or employer.

«Now we must celebrate and rejoice, because your brother was dead and has come to life again; he was lost and has been found.»

In saying these words, Jesus is denouncing a danger that you too might run into: that of wanting to live a life that is proper and respectable but which is based on seeking your own perfection and judging others to be less perfect.
In fact, if you are seeking perfection for its own sake, building yourself up without God, you are filling yourself with yourself, you are swollen with admiration for yourself. Practically speaking, you are like the son who was always at home and who enumerates to his father all his deserving merits: “All these years I served you and not once did I disobey your orders” (Lk. 15:29).

«Now we must celebrate and rejoice, because your brother was dead and has come to life again; he was lost and has been found.»

In saying these words, Jesus is opposing an attitude which sees the relationship with God as based only on observing the commandments. Observing the commandments is not enough. The Jewish tradition is also well aware of this.
In this parable Jesus puts the emphasis on divine Love and shows us that God, who is Love, takes the first step towards each person without considering his or her worthiness. He asks us to be open to him in order to establish an authentic communion of life.
You can easily understand that the greatest obstacle you can place before God who is Love is precisely a life intent on accumulating activities and works, whereas God wants our hearts.

«Now we must celebrate and rejoice, because your brother was dead and has come to life again; he was lost and has been found.»

In saying these words, Jesus is inviting you to have that same boundless love of the Father towards those whom righteous Christians would judge harshly. He is asking you not to judge with your standards the Father's love for each and every person.
By inviting the elder son to rejoice with him for the son who has been found, the Father is also asking you to change your attitude: in practice, you must welcome also as brothers and sisters the men and women towards whom you would normally nurture feelings of contempt and superiority. This will bring about a true conversion because it cleanses you from the conviction that you are better than others, it helps you avoid religious intolerance and embrace the salvation that Jesus obtained for you, as a free gift of God's love.

Chiara Lubich