2 Since these things are announced and are made plain to us, it is necessary that our obedience should wait upon the divine precepts; nor in matters of this kind can human indulgence accept any man's person, or yield anything to any one, when the divine prescription has interfered, and establishes a law. For we ought not to be forgetful what the Lord spoke to the Jews by Isaiah the prophet, rebuking, and indignant that they had despised the divine precepts and followed human doctrines.
“This people,” he says, honours me with their lips, but their heart is widely removed from me; but in vain do they worship me, teaching the doctrines and commandments of men. This also the Lord repeats in the Gospel, and says, “You reject the commandment of God, that you may establish your own tradition.” Having which things before our eyes, and solicitously and religiously considering them, we ought in the ordinations of priests to choose none but unstained and upright ministers, who, holily and worthily offering sacrifices to God, may be heard in the prayers which they make for the safety of the Lord's people, since it is written, “God hears not a sinner; but if any man be a worshipper of God, and does His will, him He hears.” On which account it is fitting, that with full diligence and sincere investigation those should be chosen for God's priesthood whom it is manifest God will hear.
Source: The Epistles of Cyprian (New Advent)