When Martin Luther wrote "The Bondage of the Will," he responded to the claim of Desiderius Erasmus who claimed that man has free will to come to God, to do the works God requires of him, and thereby to be saved. Luther firmly rejected the whole idea of free will when it comes to spiritual matters. We cannot make a move toward God; for we are dead in sin (Ephesians 2:1). We do not want to come to God, because we are, by nature, hostile to him (Romans 8:7) and enemies of God (Romans 5:10). We cannot make efforts to find God, since we are spiritually blind (2 Corinthians 4:4)
It is God who must make the move--to make us alive in Christ (Ephesians 2:4-5). It is the Holy Spirit who must enlighten us to see the glory and goodness of God (2 Corinthians 4:6). It is the Holy Spirit who must convert us, turning us from enemies of God to those who love his word, desire his will, and do good works (Titus 2:11-14). It is God who must create in us a clean heart and renew in us a right spirit (Psalm 51:10)
Luther goes on to state that God does more than just grant us new life. It is God alone who sustains that new life in us. Just as we did not give ourselves life by our own free will, so our free will does not sustain our new life in us. That, too, remains God's work.
Something from Luther on the Spirit who sustains Christian faith in us:
"So, too, I say that man, before he is renewed into the new creation of the Spirit's kingdom, does and endeavours nothing to prepare himself for that new creation and kingdom, and when he is re-created he does and endeavours nothing towards his perseverance in that kingdom; but the Spirit alone works both blessings in us, regenerating us, and preserving us when regenerate, without ourselves; as James says: 'Of His own will begat He us with the word of His power, that we should be the firstfruits of His creation' (Jas. 1.18). (James is speaking of the renewed creation.)" (page 268, The Bondage of the Will, translated by J. I. Packer and O. R. Johnston, Baker Academics: Grand Rapids, MI. (c) 1957.)
The good news is in all of this is that God does not leave it up to us to do what needs to be done for our salvation. We are weak, and we often fail. What hope could we have!? Therefore, God supplies us with our hope and confidence as he also works in us to strengthen and keep us in the true faith. While we make the effort to come to God's house to hear the word and partake in the sacraments, it is God who is at work through these to preserve us in the one true faith. And as long as we are in God's care, we are safe.