Martin Luther on Assurance

Let us not fail to thank God for delivering us from the doctrine of doubt. The Gospel commands us to look away from our own good works to the promises of God in Christ, the Mediator. The pope commands us to look away from the promises of God in Christ to our own merit. No wonder they are the eternal prey of doubt and despair. We depend upon God for salvation. No wonder that our doctrine is certified, because it does not rest in our own strength, our own conscience, our own feelings, our own person, our own works. It is built on a better foundation. It is built on the promises and truth of God.

- Martin Luther on Assurance 

Commentary on Galatians 4:6-7