The accusation was from the first pressed against Luther's theological system that it was antinomian. If the individual was to be justified by faith and not by the merits of works, then what motivation could there be for good behaviour? This accusation lead Luther to write extensively on the issue – most particularly in his Treatise on Good Works (1520). Luther was concerned to show how his ethical teaching was not an addition to his theological thought, but actually formed a unity with it.
How would Luther expound the Ten Commandments, for example? If Law and Gospel were opposed, did the Law have anything to offer the Christian believer? Luther did not want to read the Decalogue as a moral code, distinct from ceremonial or civil laws. As he pointed out, no such easy division is possible. God's law is one. All the commandments derive from the first commandment. As he said: 'the first commandment contains in itself all the other commandments. Whoever keeps it, keeps them all, and who does not keep it, neither keeps them, for his heart clings to something other than God alone.' That is: only faith can keep the fulfil the first command – and thus all the commands.
But, more crucially, Luther would insist that the law was unintelligible unless read christologically. There were two aspects to this christolological conception of the law. Firstly, the Sermon on the Mount shows that external acts are not enough to fulfil the law if inner motives are not themselves righteous. Second, there is an exposition of the law as positive commands rather than merely prohibitions.
Luther was to insist that there was no two-tier level of performance in the Christian life – works of supererogation were not taught by the gospel. The commandments come to every person as a whole – or no commandment comes at all.
Luther was not as dualistic as usually supposed. While in soteriology he did push the opposition of law and gospel, in his Genesis commentary we find him making a more subtle use of the law. Law is already present in the garden as a means of providing Adam with a way of giving concrete form to his love of God in responsive obedience. The Christian life means dying to the pride and desperation of the external code and rising again to the law in its original sense – 'the law of the spirit of life in Christ' (Rom 8:2).
Neither does Luther's moral theology rest on a dualist anthropology (as many of his interpreters and critics have supposed). In The Freedom of the Christian Luther distinguished between the 'outer and the 'inner' man. By this distinction Luther did not mean to split the individual man in two – and thus bolster make the works (or not) of the outer man an unimportant matter. Rather both concepts refer to the whole individual – the one facing God [coram deo] and the other facing the world [coram mundo]. It is not as if faith belongs to the inner and love to the outer man. Rather, love is in need of being shaped by faith. 'Faith is the 'genetrix', the fertile soil of good will and just action, in that it give a specific shape to love by transforming the affective existence of the beliver, from the 'impious affections' towards the affects of the Spirit' (Bernd Wannenwetsch).