I had a look at the Wielenberg Faith and Philosophy paper, and one problem jumps out at me. For moral properties to do the work we expect them to do, they have to be causal properties. Something being right has to at least potentially play a role in my doing what is right. But even if there are these moral properties, they aren't physical, and if all causation is physical, then these properties can't affect my behavior without violating the causal closure of the physical. In theism is true, then these moral properties can be at least possibly causally relevant. So we have, I think, still a reason for choosing a theistic account over a non-theistic one. To paraphrase Lewis, "Even if moral truths exist, what exactly have they got to do with the occurrence of a right decision as a psychological event?"
Lewis's moral argument in Miracles chapter 5 emphasizes the causal role of morality, though it is not as apparent Mere Christianity.
Wielenberg's paper is here, Lowder's recommendation of Wielenberg is here.