The claim 'God does not exist' is just as much a claim to know something as saying 'God does exist,' meaning the atheist needs just as much substantiation for his claim as the theist does for his.
If moral truths do not exist as a foundation for law, then law itself becomes merely a system of raw political power accountable to no one.
Few people can present a completely secular argument detailing why abusing toddlers is objectively wrong. But that hardly stops them from recognizing this moral truth even if they can't articulate their reasons in strictly secular terms.
What makes humans valuable in the first place? Science can't answer that question because science deals only with things we can measure empirically though the senses. If you want an answer, you'll have to do metaphysics.
To say that science is the measure of all true knowledge is not a scientific truth but a philosophic claim about science. It's scientism posing as science.
