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 (1Ti 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 (1Ti 1:9, 1Ti 1:10; compare Rom 7:7-12; Gal 3:21).