Never question the truth of this heavenly message of grace...

I love the clarity of C.F.W. Walther's words in his "Fifteenth Evening Lecture"

Unquestionably, this doctrine of doubt is the most horrible error into which a Christian can fall. For it puts Christ, His redemption, and the entire Gospel to shame. It is therefore no jesting matter.
Where are we to look for the root of this error? Nowhere else than in the commingling of Law and Gospel. Let us learn, then, rightly to divide the Word of God, the Law and the Gospel, which the Apostle Paul requires of every servant of the Church of God.
A week ago we gained the conviction that preaching the Word of God, namely, the Gospel, to a person who is sincerely alarmed over his sins, simply to call upon him to believe and apply it to himself, and never question the truth of this heavenly message of grace — that this is the only right way to give him assurance of the forgiveness of sins and a like assurance of his salvation.