Properly speaking, the Gospel is the command to believe that we have a gracious God because of Christ. ‘God sent the Son into the world, not to condemn the world, but that the world might be saved through him. He who believes in him is not condemned,’ etc. (John 3:17, 18). So whenever mercy is spoken of, faith in the promise must be added. This faith produces a sure hope, for it rests on the Word and commandments of God. If our hope were to rest on works, then it would really be unsure since works cannot still the conscience … This faith makes the difference between those who are saved and those who are not. Faith makes the difference between the worthy and the unworthy because eternal life is promised to the justified and it is faith that justifies.
(From: The Book of Concord, “The Apology of the Augsburg Confession,” (Tappert Edition), paragraphs 345-346, p. 160.)