Gregory the Great — on 1Sam 8:17 (Commentary on 1 Kings, Book 4, Chapter 2)

“9. There follows: "He will also tithe your flocks, and you shall be his servants." As if to say: When a carnal pastor is placed over you, plunder of virtues is inflicted not only upon your possessions, but also upon yourselves. For the flocks of the elect are the multitudes of spiritual virtues. But since these very spiritual virtues are scattered by the example of the wicked, the king takes tithes of the flock when he who rules carnally destroys certain virtues in the hearts of the saints. He takes tithes: because while he scatters the integrity of the mind, he leaves the number of virtues incomplete. For perfection is signified by the number ten. Hence also, when the Lord showed the losses of our fallen humanity by an open comparison, He brought forward the woman who had lost one of ten drachmas (Luke 15:4 ff.); so that by this, because the number ten is shown to have been diminished, that heavenly fellowship which remained in the angels at the number nine might be taught to be imperfect without the restoration of our condition. And because tithes are exacted each year, those who do not cease to offer tithes are fittingly said to be servants of the king. For they serve each year those by whose example they often become worse. It can also be shown through this—that after the tithes are given, this servitude is asserted—as an evil progression. For those who gradually fail act daily in such a way that they are led to the depths of iniquity. He says therefore: "He will tithe your flocks, and you shall be his servants." As if to say: By the example of the wicked you will gradually fall away, but in falling you act so that you never withdraw from imitating them. For it is written: "By whom a person is overcome, of him he is made a servant" (2 Peter 2:19). Because indeed through imitating a reprobate pastor they fall into the servitude of sin, they cannot be freed from his yoke even when they wish.”