The Formula of Concord and salvation

The Formula of Concord was written a generation after Martin Luther died. It does teach that people can lose salvation by falling away from the Christian faith. You will find references to that in the Formula of Concord. Epitome. IV. Of Good Works (Page 801 in the Concordia Triglotta) and the Formula of Concord. Thorough Declaration. IV. Of Good Works (Page 947 in the Concordia Triglotta).

Martin Luther taught from Scripture that salvation can be lost. Here is a sampling of his words: “When the Gospel begins to assert its influence, everybody wants to become a Christian. All seems well, and everybody is pleased. But when a wind or rainstorm of temptation comes on, people fall away in droves.” “The Evangelist John has put this down [the account in John 6:59-60 of many followers of the Lord stumbling over his teachings] for our comfort that it may serve us as an illustration and we may know that if the Gospel runs its true course in the world, even those fall away from it of whom one had not expected it and who ought to do their best for it. The fact that Christ acts so weakly in His ministry towards His own that one falls away here and another there has dealt me many a blow, as, on the other hand, has the fact that the devil develops such great strength and opposes the Gospel with all his might so that the best people in the world persecute it and among us also the best fall away from it completely.” [What Luther Says, Volume I, pages 37-38]

Because Christian faith is under attack, Luther offered this prayer in a hymn: “Defend your truth, O God, and stay This evil generation, And from the error of its way Keep your own congregation. The wicked ev’rywhere abound And would your little flock confound, But you are our salvation!” (Christian Worship 205:4)