"The law was added so that the trespass might increase. But where sin increased, grace increased all the more, so that just as sin reigned in death, so also grace might reign through righteousness to bring eternal life through Jesus Christ our Lord." (Romans 5:20-21)
The law did not encourage obedience, rather it magnified disobedience. Law merely indicated the sickness; grace brought about the cure.
-- Philip Yancey in What's So Amazing About Grace?