As we saw several days ago secularists find truth by finding and studying facts. For secularists truth is a description of what nature does. Truth is the correspondence between our conception of things and things as the really are. We study facts in order to refine our conception of reality. In this way our conception of reality is closer to reality as it truly exists. We simplify this concept by saying truth requires proof and proof requires fact.
Unfortunately we cannot base value on fact. We cannot say what nature ought to do merely by describing what nature does. Moral values describe what nature ought to do. Moral values are just that; values. Since we cannot base value on fact secularists have a big problem with moral truth. Since there are no moral facts there can be any moral truths.
Secularists are often accused of moral relativity. The accusation is correct. To the secularist there is fact-based truth and there is opinion. Since there can be no fact based moral truths all moral truths are nothing more than personal opinion. For them to be other than personal opinion there would have to be reference to facts that all persons could interpret in the same way. Since people who speak of moral law cannot show the truth of moral law by reference to moral fact secularists conclude there are no moral laws. In this way they defend the idea truth requires proof and proof requires fact.
Secularists adopt this position because it helps to prevent the excesses caused by using the ends to justify the means. Demanding factual proof is a way to avoid the need to settle disputes by the use of force. Given the destructive power of force in the twenty-first century one can see why its use should be avoided wherever possible. Unfortunately for secularists this also is a moral position. There are no facts to justify it.