Law tells us what to do, but also increases our desire to sin. Law uncovers sins, but offers no way out and plunges us into despair. Law produces contrition, but does not offer comfort.
The Gospel calls for faith and grants it. Gospel removes terror and gives peace in the Holy Spirit. The Gospel demands nothing of man, but transforms man, implants love in our heart, and equips us for all good works.
The Law is given to secure sinners and the Gospel to terrified sinners.