How can I do G-d's will if I don't believe in divine command theory?

St. Alphonsus de Liguori in his tract *Uniformity With G-d's Will* writes that the greatest thing in the universe is doing G-d's will. But that presupposes divine command theory: the notion that G-d first wills something, and then it becomes good because He willed it. So G-d, in an alternative universe, could have willed "thou shalt commit adultery" and it would have been good, but He didn't, so adultery just happens to be bad in this universe and you're uniting yourself to G-d's will by opposing adultery (but it could have just as easily been otherwise). How do non-believers in divine command theories of morality find happiness in doing G-d's will?