Thomas Aquinas believed the relation among natural law, Old Testament law, and New Testament law to have been well expressed by John Chrysostom’s commentary on Mark 4:28, that God “brought forth first the blade, i.e. the Law of Nature; then the ear, i.e. the Law of Moses; lastly, the full corn, i.e. the Law of the Gospel.” In other words, what is implicit in the natural law is made explicit in the Old Law and brought to glowing maturity in the New.
The practical implications of this blending of natural and Divine law are brought out in a strangely lovely way in St. Thomas’s own commentary on Christ’s remark, “But know this, that if the householder had known in what part of the night the thief was coming, he would have watched and would not have let his house be broken into” (Matthew 24:43):
“The house is the soul, in which man should be at rest. ‘When I go into my house,’ that is, my conscience, ‘I shall find rest with her [that is, Wisdom].’ The householder of the house is like the ‘king who sits on the throne of judgment,’ who ‘winnows away all evil with his eyes.’ Sometimes the thief breaks into the house. The thief is any persuasive false doctrine or temptation .... Properly speaking, the door is natural knowledge, in other words, natural right. Therefore, anyone who enters through reason, enters through its door, but anyone who enters through the door of concupiscence or irascibility or some such thing, is a thief. Thieves usually come at night. As Obadiah says, ‘If thieves came to you, if plunderers by night – how you have been destroyed!’ So if they come in the day, do not fear. In other words, temptations do not come when a man is contemplating divine things; but when he relaxes, they come. For this reason, the prophet rightly says, “forsake me not when my strength is spent.”