Jamieson, Fausset & Brown

But--"Now we know" (Rom 3:19; Rom 7:14). law is good--in full agreement with God's holiness and goodness. if a man--primarily, a teacher; then, every Christian. use it lawfully--in its lawful place in the Gospel economy, namely, not as a means of a "'righteous man" attaining higher perfection than could be attained by the Gospel alone (Ti1 4:8; Tit 1:14), which was the perverted use to which the false teachers put it, but as a means of awakening the sense of sin in the ungodly (Ti1 1:9-10; compare Rom 7:7-12; Gal 3:21).