|Makes sense to me|
A common objection is what we might call the "anything goes" objection. The idea is that if theological voluntarism were true, then God could make it the case that raping babies is morally permissible and good by commanding it, that punching your grandma in the face is a daily obligation, that microwaving cats (or better--dogs) is an act of justice, and so forth. But of course such acts are wrong--and necessarily so, some think--thus there must be something wrong with theological voluntarism.
To see why this is a bad strategy for objecting to theological voluntarism consider the following propositions:
1. If theological voluntarism were true, then raping a baby might be permissible.
2. If Kant's Categorical Imperative were true, then raping a baby might be permissible.
3. If J.S. Mill's Greatest Happiness Principle were true, then raping a baby might be permissible.
4. If moral relativism were true, then raping a baby might be permissible.
Now consider the following argument against TV and a parity argument against Kant's CI:
1. If TV were true, then raping babies might be permissible.
2. But raping babies can't be permissible.
3. Thus TV is false.
Now consider this parallel:
4. If Kant's CI were true, then raping babies might be permissible.
5. But raping babies can't be permissible.
6. Thus Kant's CI is false.
The questions that need asking are which principle or normative theory is true in the first place and which are contradictory with raping babies being permissible? (Is 1 true or is 4?) What the objector to theological voluntarism needs to show is that God could or would will that babies be raped, just as the objector to Kant's CI needs to show that raping babies is inconsistent with the CI. For it does not obviously follow that if morality is grounded in God's will that there's a possible world (or close possible world) where God wills the raping of babies.