The is-ought fallacy does not apply to christian ethics because for us the word "good" has a slightly different meaning than the unbeleiver in his or her own philosophy generaly means. Both agree that "good" actions are things we should do but we disagree over what "good" is. For the unbeleiver it is this quality or essence that is attached to ideas and actions that legitamize them being good. In short this makes "good" a self supporting concept, it takes the divine attribute of aseity. This Dooyeweerd would rightly criticize as being immanatistic philosophy, because it takes the attributes of God and gives them to creation instead of God.
For the christian "good" is a dependent concept for it totally depends on the commands and charector of God who truly has aseity and therefore can be a support for the whole of ethics. When ethics was suppossed to support itself under the modernistic project it crumbled along with nearly everything else, we have been trying to pick up the pieces eversince.