I’ll tell you something that you can reflect on and easily see: love fulfills the law. Fear of punishment does make a person act, but he acts like a slave (serviliter). If you do the good or avoid the bad because you fear you would suffer some evil, then if someone were to promise you impunity, you would immediately do the wicked thing. If you were told, “Don’t worry, nothing bad will happen to you; go ahead, and do it,” you would do it. You were called back by fear of punishment, not by love of righteousness, for love was not yet moving you.
See how love works. Let us love the one we fear and we will fear him with a pure love. A pure wife, too, fears her husband. But distinguish two loves. A pure wife fears that she will be abandoned by her absent husband; an adulterous wife fears she will be found out by her husband when he comes home. Love fulfills the law because “perfect love casts out fear” (1 Jn 4:18), servile fear, that is, the fear that comes from sin. Pure fear of the Lord lasts for age upon age” (Ps 18, 10).
If love fulfills the law, then, where does such love come from? Remember, notice, see: love is the gift of the Holy Spirit. “For the love of God has been poured out into our hearts through the Holy Spirit who has been given to us (Rom 5:5). It was proper, then, that it was after ten days–that number signifying the perfection of the law--that the Lord Jesus Christ sent the Holy Spirit because grace enables us to fulfill the law which he came to fulfill and not to destroy. (Sermon 270, 4; a Pentecost sermon)
St. Thomas Aquinas taught the same thing: “How are we to understand, ‘Where the Lord’s Spirit is, there is freedom’? A free person is one who is his own, while a servant is one who belongs to another. Anyone who acts on his own, then, is free, while one who acts because moved by another is not acting freely. A person who avoids evil, then, not because it is evil but because God commands it, is not free, while someone who avoids evil because it is evil is free. This is what the Holy Spirit does who inwardly perfects the mind with a good habit so that a person avoids evil out of love as if the divine law were ordering it. Such a person is said to be free, not because he is not subject to the divine law but because he is inclined by his good habit to do what the divine law commands. (Commentary on 2 Cor 3:17)
These texts are important in response to those who regularly counterpose divine authority and human autonomy or who make obedience even to divine command the supreme or even the only virtue. It’s love that makes all the difference.