4 In order, therefore, that we may remain in the unity of the faith, and not, stumbling at the offenses occasioned by the chaff, desert the threshing-floor of the Lord, but rather remain as wheat till the final winnowing, and by the love which imparts stability to us bear with the beaten straw, our great Shepherd in the gospel admonishes us concerning the good shepherds, that we should not, on account of their good works, place our hope in them, but glorify our heavenly Father for making them such; and concerning the bad shepherds (whom He designed to point out under the name of Scribes and Pharisees), He reminds us that they teach that which is good though they do that which is evil.
Source: Letters (New Advent)