Perseverance, Lutheran Confessions

"Fourthly, The entire [the holy Catholic, Christian] Church confesses that eternal life is attained through mercy. For thus Augustine speaks, De Gratia et Libero Arbitrio, when indeed he is speaking of the works of the saints, wrought after justification: "God leads us to eternal life not by our merits, but according to his mercy." And Confessions, Book ix.: "Woe to the life of man, however much it may be worthy of praise, if it be judged with mercy removed." And Cyprian in his treatise on the Lord's Prayer: "Lest anyone should flatter himself that he is innocent, and by exalting himself, should perish the more deeply, he is instructed and taught that he sins daily, in that he is bidden to entreat daily for his sins." But the subject is well known, and has very many and very clear testimonies in Scripture, and in the Church Fathers, who all with one mouth declare that even though we have good works, yet in these very works we need mercy."
Ap VI 201

Accordingly Daniel also prays (9:18, sq.): "For we do not present our supplications before thee for our righteousnesses, but for thy great mercies. O Lord, hear; O Lord, forgive; O Lord, hearken and do it; defer not for thine own sake, O my God; for thy city and thy people are called by thy name." Thus Daniel teaches us in praying to lay hold upon mercy, i.e. to trust in God's mercy, and not to trust in our own merits before God. We also wonder what our adversaries do in prayer, if, indeed, the profane men ever ask anything of God. If they declare that they are worthy because they have love and good works, and ask for grace as a debt, they pray precisely like the Pharisee in Luke 18:11, who says: "I am not as other men are." He who thus prays for grace, and does not rely upon God's mercy, treats Christ with dishonor, who, since he is our high priest, intercedes for us.
210, 211

"Let us, therefore, hold fast to this which the Church confesses, viz. that we are saved by mercy."
223

It is a great temptation for those who have been justified by faith, to move from faith alone and include works for justification. It is even taught by some in such a way that works serve to complete faith, rather than follow as an adornment. They therefore judge their justification, or state of grace, according to their performance of whatever works they have set up as necessary, for such works are evident to them while the promise of God in the gospel is unseen and cannot be grasped or measured by works.  However, we must remember that we are forgiven and justified by grace thru faith alone in the merits of Christ. It then follows that where this justifying faith is present, good works will also be found, but since the saved and justified still sin much every day, mercy alone preserves us as God leads us by the Spirit and raises us up again when we fall. It is purely pharisaical to say that God preserves in faith by the Spirit those who are able to refrain from sinning to such a degree that God must abandon them, for they are then praying as did the Pharisee, "I thank you God that I am not as other men are." We are not forgiven by mercy and then made to retain this forgiveness according to works. Rather, we are forgiven by mercy and sustained in this forgiveness by mercy in spite of our many sins. Yet it does not follow that true faith abides with impenitence.