Second, dear Mother, you also know the true center and foundation of your salvation from whom you are to seek comfort in this and all troubles, namely, Jesus Christ, the cornerstone. He will not waver or fail us, nor allow us to sink or perish, for he is the Savior and is called the Savior of all poor sinners, and of all who are caught in tribulation and death, and rely on him, and call on his name.
[Christ] says: “Be of good cheer; I have overcome the world.” If he has overcome the world, surely he has also overcome the sovereign of this world with all his power. But what else is [the devil’s] power but death, by which he has made us subject to himself, [and] held us captives on account of our sin? But now that death and sin are overcome, we may joyfully and cheerfully listen to the sweet words: “Be of good cheer; I have overcome the world.” We certainly are not to doubt that these words are indeed true. More than that, we are commanded to accept this comfort with joy and thanksgiving. Whoever would be unwilling to be comforted by these words would do the greatest injustice and dishonor to the dear Comforter, as if it were not true that he bids us to be of good cheer, or as if it were not true that he has overcome the world. [If we acted thus,] we would only restore within ourselves the tyranny of the vanquished devil, sin, and death, and oppose the dear Savior. From this may God preserve us. (Luther’s Works, v. 50, p.19)