John Chrysostom

For the father himself rejoices in the return of his son, and feasts on the calf, because the Creator, rejoicing in the acquisition of a believing people, feasts on the fruit of His mercy by the sacrifice of His Son. Hence it follows, For this my son was dead, and is alive again.

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

So then, everyone who repents and becomes a son of God, and especially one who is restored and generally cleansed from sin, partakes of this fatted Calf and becomes a cause of joy for the Father and His servants, the Angels and priests: "For he was dead and is alive again, was lost and is found." In the sense that he remains in evil, he "was dead," that is, without hope, but in the sense that human nature is easily inclined and can turn from wickedness to virtue, he is called "lost." For the word "lost" is milder than "dead."

Commentary on Luke ↗

Augustine of Hippo

Those banquets are now celebrated, the Church being enlarged and extended throughout the whole world. For that calf in our Lord's body and blood is both offered up to the Father, and feeds the whole house.

Catena Aurea by Aquinas ↗

Bede

And they began to celebrate. These feasts and festivities are now celebrated, extended and spread throughout the world by the Church. For indeed that calf is offered in the body and blood of the Lord, both to the Father, and it feeds the whole household.

On the Gospel of Luke ↗

Bonaventure

Secondly, as regards the consideration of the man who is saved, he adds: Because this my son was dead, through sin, which makes men dead: Psalm: "The dead shall not praise you, O Lord"; and again John 8: "You shall die in your sin." And has come back to life, through repentance: Ephesians 5: "Rise, you who sleep, and arise from the dead, and Christ shall enlighten you"; and Psalm: "You shall take away their spirit, and they shall fail." "Send forth your spirit," etc. …

Commentary on Luke, Chapter 15 ↗