As a dog returns to his vomit, so a fool returns to his folly.
Read Chapter 26
Caesarius of Arles
These, no doubt, are the things suffered by clerics, monks or virgins who are proud, disobedient and lukewarm. When at the beginning of their life they abandoned the ways of this world and with a fervent spirit fled to the service of holy religion, through the grace of God they were rid of all their sins. But afterwards, when they did not put forth zeal because of carelessness and sloth and were not filled with spiritual graces through the help of God, the vices which had departed found them empty and returned with many more and compelled them to return to their vomit. Then was fulfilled in them what is written, “As the dog that returns to his vomit becomes hateful, so is the sinner that returns to his sin.”
We have learned that in some less observant monasteries, men have entered and brought their capital with them and later, losing their religious fervor, have made great trouble in demanding their property. Returning to the world which they had left, as dogs return to their vomit, with the aid of their relatives they have extorted what they had brought with them to the monastery and have sought the support of secular judges. With the help of magistrates they have [thus] destroyed the monasteries, so that we see many innocent men ruined by a single sinner.
Dog. This is the only animal which is known to do so. St. Peter uses this comparison to deter any from renouncing the faith; as the Fathers do, to show the misery attending a relapse. Septuagint here add, "there is a confusion", taken from Ecclesiasticus iv. 25. (Calmet)
Don’t you know that people so unconcerned about their own salvation and vacillating between attention to it and headlong course into the devil’s net are compared in sacred Scripture with dogs? It says, remember, “The person who turns away from his sin and then goes back to it is like a dog returning to its vomit.”