Martin Luther decried the doctrine of doubt in his day saying, "It is intolerable to declare in Christendom that we cannot and must not know whether God is gracious to us."
Instead, Luther points us to the firm assurance found in Christ alone when he writes, "He who wants to be saved and go to heaven when he dies must think and say: 'Have mercy on me, gracious God. I am a poor, sinful being who has merited nothing but wrath. But whether my life was good or evil, I know that I need not doubt that I was baptized and named a Christian for the remission of sins; that Christ my Lord was born, suffered, died, and was raised from the dead for me; that He gave me His holy body and blood as food for the strengthening of my faith; and that I have been absolved and relieved of my sin in the name and by the power of Christ.' Such a heart and such a faith cannot fare adversely or be lost any more than God’s Word can fail or be false. This I can guarantee you, since God Himself guarantees it to you through His Word" (Luther's Works, Vol 24, p.222).
We will find nothing but doubt, darkness, and doom when we look to our own goodness and works in order to obtain salvation. When we are focused on God's mercy in Christ, then the confidence of our salvation is as firm as Christ is gracious.