In his book Christian Ethics in the Modern Age, British philosopher-theologian Brian Hebblethwaite offers a nice summary of what I tend to think of as the classic Christian understanding of the nature of ethics:

Christians certainly believe that all goodness stems from God and reflects both God’s own nature and His will for man. But recognition of this comes in two ways: the good for man is built into human nature and can be discerned, however fragmentarily and incompletely, in what makes for human relationship and human flourishing. This ‘natural’ recognition of the good can be affirmed despite the ‘fallen’ state of man. But the true good for man is further revealed, so Christians believe, through the saving acts of God, culminating in the story of Jesus and his Resurrection. (p. 13)

Hebblethwaite goes on to point out that both of these ways of knowing are subject to distortion and in need of correction:

Here too there is no guarantee of freedom from distortion in the human media of revelation or in man’s understanding of divine revelation. So it is quite possible for the two channels of moral knowledge, human experience of goodness and human response to the revelatory acts of God, mutually to illuminate and correct each other. Moral criticism of religious revelation-claims is possible because natural human morality is itself a reflection of the image of God in man. Christian morality’s criticism and enhancement of natural human morality are possible since they reflect the definitive revelation of God’s nature and will through His saving acts.

And yet there is only one moral truth:

But the two channels, on this view, cannot be ultimately incompatible, since it is the same divine nature that is reflected, however hazily, in human goodness, as is reflected most clearly, on the Christian view, in the character of Christ. But the divine revelation, including the character of Christ, has itself to be understood and applied correctly by men and women down the ages; they may get it wrong, and thus be open to moral criticism.

Where Hebblethwaite may depart from some classical views is in recognizing the context-bound nature of our understanding of revelation. Even if God reveals his will and nature to us, that revelation still has to be mediated through human language, concepts, and understanding. These will (inevitably?) produce a kind of distortion. This is why Hebblethwaite can say that our natural morality can act as a corrective on “revelation”: because no revelation is sheerly self-authenticating. A purported revelation that outright contradicted some tenet of natural morality would for that reason be highly suspect.

This also allows Hebblethwaite to say that Christian morality can be not only confirmed, but enriched by insights from other sources. We can see, both in other religions and in secular movements, aspects of God’s goodness reflected that may not have been so clear in our own tradition. Christians may be compelled to reject insights that flatly contradict the Christian revelation, but they needn’t believe that they all ready possess the truth, the whole truth, and nothing but the truth. For instance, it seems clear that Christian theology and practice have been enriched by the insights of such movements as feminism, Marxism, and environmentalism, insights that are compatible with the essential tenets of Christianity, but which Christians previously weren’t able to derive from the resources of their own tradition. Similarly, other religions can reveal aspects of the truth that Christians might not otherwise have been aware of.

On this view, explicit belief in God is not necessary to discover the conditions of human flourishing, but those conditions are–ontologically speaking–rooted in or derived from God’s creative will and goodness. Thus the revelation of that will in Christ gives a fuller and clearer picture of the good. Just as importantly, Hebblethwaite argues, the work of Christ, the sending of the Spirit, and the formation of the Church provide resources for embodying that good in our lives and communities that transcend natural human capabilities.