The Bondage of the Will

In 1524, Erasmus — the most famous scholar in Europe, and no friend of Rome either — published a careful, moderate treatise arguing that human beings retain some freedom to cooperate with God's grace. Luther took a year to answer. When he did, it was not moderate.

Late in his life, asked which of his own books were worth keeping, Luther named only two: the Small Catechism, and this one. Everything else he considered occasional writing. This is the argument he thought actually mattered: that in the matter of salvation, the human will contributes nothing, and every claim otherwise quietly takes credit that belongs to God alone.

It is long, it is repetitive by design, and it does not treat Erasmus gently. Read it as the fullest statement of a claim the rest of Lutheran theology assumes rather than argues for.

Where to start

This is not a text to start Lutheranism with. Read the Small Catechism and the Freedom of a Christian first. Come back to this once you already know what Luther means by grace, so you can see what he thought was at stake in denying it any human contribution.