Objection 1. It would seem that not all are subject to the law. For those alone are subject to a law for whom a law is made. But the Apostle says (1 Timothy 1:9): "The law is not made for the just man." Therefore the just are not subject to the law.
Objection 2. Further, Pope Urban says [Decretals. caus. xix, qu. 2]: "He that is guided by a private law need not for any reason be bound by the public law." Now all spiritual men are led by the private law of the Holy Ghost, for they are the sons of God, of whom it is said (Romans 8:14): "Whosoever are led by the Spirit of God, they are the sons of God." Therefore not all men are subject to human law.
Objection 3. Further, the jurist says [Pandect. Justin. i, ff., tit. 3, De Leg. et Senat.] that "the sovereign is exempt from the laws." But he that is exempt from the law is not bound thereby. Therefore not all are subject to the law.
On the contrary, The Apostle says (Romans 13:1): "Let every soul be subject to the higher powers." But subjection to a power seems to imply subjection to the laws framed by that power. Therefore all men should be subject to human law.
I answer that, As stated above (90, A1,2; 3, ad 2), the notion of law contains two things: first, that it is a rule of human acts; secondly, that it has coercive power. Wherefore a man may be subject to law in two ways. First, as the regulated is subject to the regulator: and, in this way, whoever is subject to a power, is subject to the law framed by that power. But it may happen in two ways that one is not subject to a power. In one way, by being altogether free from its authority: hence the subjects of one city or kingdom are not bound by the laws of the sovereign of another city or kingdom, since they are not subject to his authority. In another way, by being under a yet higher law; thus the subject of a proconsul should be ruled by his command, but not in those matters in which the subject receives his orders from the emperor: for in these matters, he is not bound by the mandate of the lower authority, since he is directed by that of a higher. In this way, one who is simply subject to a law, may not be a subject thereto in certain matters, in respect of which he is ruled by a higher law.
Secondly, a man is said to be subject to a law as the coerced is subject to the coercer. In this way the virtuous and righteous are not subject to the law, but only the wicked. Because coercion and violence are contrary to the will: but the will of the good is in harmony with the law, whereas the will of the wicked is discordant from it. Wherefore in this sense the good are not subject to the law, but only the wicked.
Reply to Objection 1. This argument is true of subjection by way of coercion: for, in this way, "the law is not made for the just men": because "they are a law to themselves," since they "show the work of the law written in their hearts," as the Apostle says (Romans 2:14-15). Consequently the law does not enforce itself upon them as it does on the wicked.
Reply to Objection 2. The law of the Holy Ghost is above all law framed by man: and therefore spiritual men, in so far as they are led by the law of the Holy Ghost, are not subject to the law in those matters that are inconsistent with the guidance of the Holy Ghost. Nevertheless the very fact that spiritual men are subject to law, is due to the leading of the Holy Ghost, according to 1 Peter 2:13: "Be ye subject... to every human creature for God's sake."
Reply to Objection 3. The sovereign is said to be "exempt from the law," as to its coercive power; since, properly speaking, no man is coerced by himself, and law has no coercive power save from the authority of the sovereign. Thus then is the sovereign said to be exempt from the law, because none is competent to pass sentence on him, if he acts against the law. Wherefore on Psalm 50:6: "To Thee only have I sinned," a gloss says that "there is no man who can judge the deeds of a king." But as to the directive force of law, the sovereign is subject to the law by his own will, according to the statement (Extra, De Constit. cap. Cum omnes) that "whatever law a man makes for another, he should keep himself. And a wise authority [Dionysius Cato, Dist. de Moribus] says: 'Obey the law that thou makest thyself.'" Moreover the Lord reproaches those who "say and do not"; and who "bind heavy burdens and lay them on men's shoulders, but with a finger of their own they will not move them" (Matthew 23:3-4). Hence, in the judgment of God, the sovereign is not exempt from the law, as to its directive force; but he should fulfil it to his own free-will and not of constraint. Again the sovereign is above the law, in so far as, when it is expedient, he can change the law, and dispense in it according to time and place.
Source: Summa Theologica (New Advent)