Scripture shows us what this true ought is and in societies where Christianity has not yet had significant influence the distinction between Christians and non-Christians in their approach to morality can be quite marked.
The general sense of ought in a person is a pointer to God. If God exists then there is the possibility for an objective morality; the objectiveness coming from God's character. It is objective because it is obligatory and applies to all men. The obligation comes because we are owned by God.
If God is non-existent then there can be no objectivity. There is no true obligation. There is nothing that allows us to say this is how it should be for all men. All is personal opinion and competing preferences. But a personal preference is not an "ought", it is preference. A preferred behaviour is the same as a preferred food or a favourite tie.
So without God how do we explain this universal trait? And even if an explanation is forthcoming it does not answer the question of why we should obey the "ought." If we recognise it as a quirk of nature then we are under no obligation to obey it, even if we can explain its reason for coming into existence. So without God, that is, without objectivity we realise that this is not true morality, it is merely an apparent morality. Something that confers survival value but it derives its importance from what it offers, not what it intrinsically is.
However this is not how we see morality. When we examine ourselves we know it is not a preference. In us there is a sense of objectivity. Not all moral codes are equal. Some choices really are better than others. We may argue morality is subjective in the abstract but when faced with certain behaviour it becomes clear that not all is equal. We appeal to some standard which must be external to us. By judging competing systems and claiming some systems are better than others we are appealing to objectivity. Objectivity can only come with a moral God.
- If there is no God then morality is subjective
- If there is a God then morality may be objective
- If morality is objective then there must be a God.