More Luther on how to endure trials and temptations, especially to despair:

This picture of the conflicts and struggles in the saints is full of consolation. Elsewhere the example of the nun Mechtild is recounted. She was vexed by the devil, because she knew or experienced absolutely nothing about faith. This was a temptation to unbelief, which is a most bitter grief and torment of conscience. For hearts are consumed by trepidation and doubt, and experience alone shows what this grief is; it cannot be declared in words. Nevertheless, that temptation was not yet equal to this struggle of Jacob. For it was not God who was fighting against her, as was the case here with Jacob, but the devil, who can drive men to unbelief, despair, and blasphemies against the Holy Spirit. I myself, for example, have seen some, and especially women, who simply complained that they were damned and cast off, because they were vexed by Satan with the spirit of blasphemy.

Now, the nun held nothing else up against this temptation except this statement: “I AM A CHRISTIAN,” that is, “I have been baptized in the blood of God’s Son; I have been fed with the body and blood of Christ. These things I firmly cling to; with this consolation I am content, even if God Himself were to speak otherwise.” Similarly, Abraham was ordered to kill the son of promise, and yet he believed very firmly that God would keep His promises.

These matters must be dealt with carefully for the sake of those who will be future pastors of the churches. For there will always be some who will suffer these temptations. They should be cheered up and strengthened by the voice of the pastors in this manner: “In like manner, have confidence, my son; believe that you have been baptized, that you have been pastured and fed in the Lord’s Supper and absolved by the laying on of hands, not mine, but God’s, who has said to you: ‘I forgive you your sins; I promise you eternal life.’ ” If they grasp this in firm faith, temptations and the spirit of blasphemy will vanish.

AE 6, on Genesis 32