But God's Law cannot save us from sin and death. Only the grace of God--the free gifts of relationship with Him, forgiveness of sins, and new life given to all who believe in Jesus Christ--can save us. The Law can only show us of our need of the grace that saves us.
I love this paragraph from Martin Luther's comments on Galatians 3:30, where he explains all of this:
The proverb has it that Hunger is the best cook. The Law makes afflicted consciences hungry for Christ. Christ tastes good to them. Thirsty souls are what Christ wants. He invites them: "Come unto Me, all ye that labour and are heavy laden, and I will give you rest." Christ's benefits are so precious that He will dispense them only to those who need them and really desire them.