Voluntarism is the idea that all that exists, including abstract notions, rules of logic, mass, the speed of light, geometrical axioms, etc., are all caused by God and could therefore have been caused to be other than they are. This includes that God could have caused himself to be incapable of causing anything in any other way than he caused it, as well that he could have caused kangaroos to be both blue and pink in the same way at the same time with all the same conditions, i.e., he could have willed contradictions. This is believed to follow from his omnipotence. Thus a voluntarist's answer to the old question "Could God make a rock so big that he could not lift it?" is "Yes, of course."
Voluntarists are very difficult to argue with. If I suggest a line of reasoning, they are at full liberty to respond that God could very well have caused whatever logical principles I am employing to not hold in his own case. Thus, my argument is to use that very strength against them, since there are some things which we as humans tend to hold very tightly to.
Suppose God is as the Voluntarists, i.e., he is a voluntarist God, what appears to follow? Well, I have no reason to suppose any rule to work reliably, apart from God's grace. God could very well cause gravity to only work on those objects which have not chewed dental floss at midnight while balancing on a tightrope, and could cause this to be necessary given his character without altering his character. Now, then, if God is a voluntarist God, what becomes of science? If we say that God is good and therefore causes the physical laws to be consistent then I ask how we know that God has caused that to be good. We no longer have any way of getting anywhere by means of logic, and thus no way of knowing God. Not even the Bible avoids logic enough for us to simply read it and thereby know God, if he is a voluntarist God, since he could have caused the words to say false things and still be totally and completely inerrant. Thus a human has no basis on which to claim that they have come to some sort of truth, for any basis could have been caused, without it itself changing, to be a perfectly good reason to say that we had fallen into falsehood.
Thus, voluntarism makes it impossible to have any reasons for anything, including itself, and is thereby just as self-refuting as moral relativism.