There is no question that Andrew Fuller advocated an account of faith and of preaching that returned it to Reformation principles; a general, free, indiscriminate offer of the Gospel, and justification by faith as reliance upon the Christ preached. But fresh controversies lead to new emphases. As Heraclitus said, it is impossible to step into the same river twice.
Fuller placed a good deal of emphasis on faith as a duty, and his views came to be known as ‘duty faith’ and ‘Fullerism’, terms suggesting something distinctive. In the book he returns to the matter several times. Most interesting is his discussion in his Concluding Reflections. Here he seems to qualify his earlier emphasis. He recognizes here that faith cannot be a duty straightforwardly, since the invitations of grace are not founded merely on divine authority, or on God’s goodness, but in particular on God’s mercy and grace. He here emphasizes the important qualification, that ‘Though believing in Christ is a compliance with a duty, yet it is not as a duty, or by way of reward for a virtuous act, that we are said to be justified by it….we must stand accepted in the Beloved’. Yet even with this vital qualification, we may ask, is faith as a compliance with a duty a dominant note in the New Testament? Sinners may have a duty to repent but do they, in the same sense, have a duty to believe?
Perhaps Fuller was at his most nervous here, at the very heart of his position, as is shown by the fact that he discusses this matter first at proposition III of Part II, then in his Answers to Objections, and finally in his Concluding Reflections. Fuller says that though the Gospel is a message of pure grace it ‘virtually requires’ obedience. But if something virtually requires obedience then it requires obedience. I think that by virtual obedience Fuller means that, like the law, the invitations of the gospel call for our compliance, yet they are not as the law is, divine commands, but the gospel invitations. That's my guess.
Immanuel Kant contrasted the sense of duty with what he called 'a holy life'. If we do certain acts as duties, dutifully, the requirements in question bear down upon us. We feel pressured to do what a part of us, perhaps the whole of us, does not want to do. We have to do it. Our action is (Kant said) heteronomous. If, somehow, we have internalised the norms we discern in the commands, if we want to do them in the core of our person, then, Kant also said, we possess (in his sense) a holy life. We act autonomously. We no longer have any duties. Or to the extent that we internalise the norms, in this sense we possess a holy life. (I don't think Kant said this bit, on reflection, but he may have.) We do the substance of what are duties but not as duties. ('Whose service is perfect freedom'.)
Paul’s habitual way of teaching us how to behave is to emphasise that Christians are new people – ‘How can we who died to sin still live in it?’ , ‘Put off the old self….be renewed in the spirit of your minds….put on the new self, created after the likeness of God in true righteousness and holiness’….'If then you have been raised with Christ, seek the things which are above, where Christ is'….'For we are all children of light, children of the day…so then let us not sleep as others do, but let us keep awake and be sober….'And he writes of gifts, graces, and Peter of virtues. Lists of them.
If we ask, how should we then live, the answer is, by the norms and values as expressed in the Decalogue, what Calvin called ‘the eternal law’, expressed, he thought, in the Golden Rule. But (ideally) not as duties, even though in our present far-from-perfect state, expressing such norms in living - as distinct from talking about them – may be irksome and difficult. But for new men and women in Christ 'doing our duty' is not quite the best way of expressing this, is it? Yet if all else fails then duty it had better be.