The biggest stumbling block for many Reformed Christians in Lutheran theology is the idea that a believer can fall away from faith and lose salvation. It seems, by the account of some, to deny that salvation is all God's work and not ours.
First let me outline the Lutheran view of perseverance. God and God alone initiates regeneration and gives man faith. Man is completely passive in conversion. He merely receives what God gives. God sanctifies man, continually causing him to grow in his hatred for sin and love of righteousness. God does this through means. These means are word and sacrament. These elements must be present or faith will not be sustained. Man does not "cooperate with God" in sanctification. It is wholly the Spirits work. He can however, reject the gift of faith, especially if he avoids the God-given means of sanctification; namely, word and sacrament. This is not the Wesleyan idea, wherein one must continually be afraid of doing enough good works, or doing a bad deed which will cause him to lose his salvation. We are not in the constant process of going in-and-out of grace. However, man can fall away from grace if he has embraced the way of the flesh as opposed to that of the Spirit and lost faith.
This idea is shown by the falling away passages shown through out the New Testament. Even in the great chapter of assurance, Romans 8, there is a hint of this. "The Spirit Himself bears witness with our spirit that we are children of God, and if children then heirs- heirs of God and fellow heirs with Christ provided we suffer with Him in order that we also may be glorified with him." (Romans 8:16-17) There is a conditionality in this text. Our faith must be sustained through suffering or we will not be glorified. Our glorification is conditional upon our suffering.
In the book of Colossians, Paul gives great assurance to his readers through the work of Christ. However, at the end of this discussion he makes an interesting statement. "And you, who were once alienated and hostile in mind, doing evil deeds, he has now reconciled in his body of flesh by his death, in order to present you holy and blameless and above reproach before him, if indeed you continue in the faith, stable and steadfast, not shifting from the hope of the gospel that you heard, which has been proclaimed in all creation under heaven, and of which I, Paul, became a minister." (Colossians 1:21-23)
We shall only be presented before him as holy and blameless if we stay in the faith. The clear implication of this passage is that some may indeed shift from the hope of the gospel. It does not do justice to the text to make this simply hypothetical.
Hymenaeus and Alexander seem to be another example of those who have fallen away from faith. "By rejecting this, they have made a shipwreck of their faith, among whom are Hymenaeus and Alexander, whom I have handed over to Satan that they may learn not to blaspheme." (1 Timothy 1:19-20) Paul later describes others who will fall away from the faith, "Now the Spirit expressly says that in later times some will depart from the faith..." (1 Timothy 4:1)
Hymenaeus and Alexander are clearly away from the faith as Paul accuses them of blasphemy. He does however hold out hope that God may bring them back.
Jesus in his parable of the sower seems to assume that man can fall away from grace. "And the ones on the rock are those who, when they hear the word, receive it with joy. But these have no root; they believe for a while, and in time of testing fall away. And as for what fell among the thorns, they are those who hear, but as they go on their way they are choked by the cares and riches and pleasures of life, and their fruit does not mature. As for that in the good soil, they are those who, hearing the word, hold it fast in an honest and good heart, and bear fruit with patience." (Luke 8:13-15) The problem is not that those on the rock or among the thorns do not have faith, it is that their faith is not enduring as are those in good soil. Jesus admits that they believe, though this faith will not last until the end.
Hebrews has been the book that presents the most problems for the Calvinistic position on perseverance. The purpose of the book is that the author is encouraging Jews who have been converted not to fall back into Judaism. The assumption is that falling away is possible. I will just quote a few passages to make my point.
"Take care brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God. But exhort one another every day, as long as it is called 'today' that none of you may be hardened by the deceitfulness of sin. For we have come to share in Christ, if indeed we hold our original confidence firm to the end." (Hebrews 3:13-14) To fall away from the living God is to assume that one was once in a relationship with the living God. He encourages those in the church to encourage one another so that they may not fall away. Notice that there is conditionality in final salvation similar to that found in Colossians. The condition is that one's faith and trust remain. This theme continues, "For it is impossible in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt." (Hebrews 6:4-6) These people described can fall away to such a point where they will not ever be brought back to repentance. They clearly are christian individuals because they have "shared in the Holy Spirit." There is no Biblical precedence for seeing unbelievers having shared in the Holy Spirit. Finally, Hebrews 10 repeats the same point, "For if we go on sinning deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a fearful expectation of judgment, and a fury of fire that will consume the adversaries." (Hebrews 10:26) These are believers who have "received the knowledge of the truth." Later, the author states that they have been sanctified by the blood of the covenant.