Jeremiah 35:18 - Ellicott's Commentary On The Whole Bible

Because ye have obeyed the commandment of Jonadab your father. — The words decide nothing as to the obligation of the commandment referred to upon others. The law which Jeremiah received as given by God laid down no such rule of life. A righteous life was possible without it (Jeremiah 22:15; Matthew 11:19). What he was taught to praise was the steadfastness and loyalty with which they adhered to a merely human precept, not at variance with the letter of any divine law, and designed, like the Nazarite vow, to carry the spirit of that law — the idea of a life-long consecration — to its highest point. The temper of faithfulness to any rule of life sanctioned by prescription, whether it be that of a school, a college, a guild, or a religious order, is in itself praiseworthy as compared with that of individual self-assertion and self-will.