Ambrose of Milan — as excerpted in the Catena Aurea on Luke 9:57-62

“Or, He compares foxes to heretics, because they are indeed a wily animal, and, ever intent upon fraud, commit their robberies by stealth. They let nothing be safe, nothing be at rest, nothing secure, for they hunt their prey into the very abodes of men. The fox again, an animal full of craft, makes no hole for itself, yet likes to lie always concealed in a hole. So the heretics, who know not how to construct a house for themselves, circumscribe and deceive others. This animal is never tamed, nor is it of use to man. Hence the Apostle, A heretic after the first and second admonition reject. (Tit. 3:10.) But the birds of the air, which are frequently brought in to represent spiritual wickedness, build as it were their nests in the breasts of the wicked, and as long as deceit reigns over the affections, the divine principle has no opportunity to take possession. But when a man has proved his heart to be innocent, upon him Christ leans in some measure the weight of His greatness, for by a more abundant shedding of grace He is planted in the breasts of good men. So then it does not seem reasonable that we should think him faithful and simple, who is rejected by the judgment of the Lord, notwithstanding that he promised the service of unwearied attendance; but our Lord cares not for this kind of service, but only purity of affection, nor is his attendance accepted whose sense of duty is not proved. For the hospitality of faith should be given with circumspection, lest while opening the interior of our house to the unbelieving, through our imprudent credulity we fall a snare to the treachery of others. Therefore that you may be aware that God despises not attendance upon him but deceit, He who rejected the deceitful man chose the innocent. For it follows, And he said unto another, Follow me. But He says this to him, whose father He knew to be dead. Hence it follows, But he said, Lord, suffer me first to go and bury my father.”