Sin and repentance

I have been searching through Martin Luther's writings concerning the possibility of losing one's salvation. I am having a hard time finding anything that says that one can lose the salvation bought by Christ. I am certainly not speaking of the Calvinistic idea that once God has given the free gift of salvation through His grace and mercy, one is free to do as one pleases and sin may abound without consequence. I speak of the idea that one may lose salvation by error and sin. How does one then return to a state of grace? Sinful humans sin within the first ten minutes of each new day, so how can one be assured of salvation? I am unable to find this in the writings of Martin Luther that I am familiar with. Please direct me to some materials so I may have confidence in this issue. I have been an LCMS Lutheran for 15 years now and am not finding sufficient answers. Thank you for taking the time for my question.

In volume one of What Luther Says, you can read statements like these in the “Apostasy” section:

“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.”

“Stand fast, then, hold on, do not flee, do not draw back. If you have begun to believe, carry it through to the end. There are many who stay with it, shed their blood and boldly stake and risk everything. There are the true disciples, and they remain constant. But ten times greater was the number of those who began to believe with us and whom our doctrine at first pleased, but not ten percent have remained faithful till now.”

“When the devil gets a man into his clutches who has been in our midst and also has the Bible, such an apostate is worse and more harmful than all the heathen, who know not Scripture.”

Through impenitence and unbelief, Christians can forfeit the salvation they once enjoyed. On the other hand, Christians do not veer out of faith and come back into it with their daily sins of weakness and their repentance. As Christians, we are living in a state of being forgiven: “Therefore, since we have been justified through faith, we have peace with God through our Lord Jesus Christ, through whom we have gained access by faith into this grace in which we now stand” (Romans 5:1-2). That passage assures us that we stand in grace, that is, we live our lives as Christians trusting that we are forgiven through Jesus Christ. We are assured of salvation because we have God’s own word on it (John 3:16).