Another way to address this is to think of "absolute morality" as a top-down process.
We start with a perfect model of absolute perfect goodness - god - and everything else is measured in comparison to that. It is reminiscent of Platonic Essentialism where every triangle you have ever seen is an imperfect copy of the ideal triangle hanging somewhere in abstract space. The Platonic geometer might fear that if his essential triangle did not in fact exist he could never assess triangles in the real world - just as the theist fears that without god we can never judge anything to be good or evil
Both are wrong.
We can faithfully produce triangles, or any other shape, using simple geometric principles without reference to Platonic ideals. And we can make objective moral judgements by applying the tools our evolved minds possess to reason on the effects of our actions.