But we know that the law is good - The law as given by God, is both good in itself and has a good tendency. This is similar to what the apostle had asserted, Romans 7:12-16; : The law is holy; and the commandment is holy, just, and good; see the note on Romans 7:12, etc.
If a man use it lawfully - That is, interpret it according to its own spirit and design, and use it for the purpose for which God has given it; for the ceremonial law was a schoolmaster to lead us unto Christ, and Christ is the end of that law for justification to every one that believes. Now those who did not use the law in reference to these ends, did not use it lawfully - they did not construe it according to its original design and meaning.
But we know that the law is good - We admit this; it is that which we all concede. This declaration is evidently made by the apostle to guard against the supposition that he was an enemy of the law. Doubtless this charge would be brought against him, or against anyone who maintained the sentiments which he had just expressed. By speaking thus of what those teachers regarded as so important in the law, it would be natural for them to declare that he was an enemy of the law itself, and would be glad to see all its claims abrogated. Paul says that he designs no such thing. He admitted that the law was good. He was never disposed for one moment to call it in question. He only asked that it should be rightly understood and properly explained. Paul was never disposed to call in question the excellency and the utility of the law, however it might bear on him or on others; compare Romans 7:12 note, and Acts 21:21-26 notes.
If a man use it lawfully - In a proper manner; for the purposes for which it was designed. It is intended to occupy a most important place, but it should not be perverted. Paul asked only that it should be used aright, and in order to this, he proceeds to state what is its true design.