Do Christians derive an obligation from nature? How do we bridge the is/ought gap? It must be admitted the naturalistic fallacy is an informal fallacy thus there can be exclusions to the rule (e.g. an explanatory ultimate of any given ethical theory). How do Christian ethics deal with this fallacy?
We do not derive an 'ought' from an 'is' in the sense we start without moral values and duties then derive them from nature (e.g. Physics or Chemistry). However, we do concede we derive an 'ought' from an 'is' with good reason. We start with God's intrinsic value, He chose to create man in His image. We may argue:
(1) If God created all mankind in His image with contingent intrinsic infinite moral value and worth then moral values and duties are properly basic.
(2) God created all mankind in His image with contingent intrinsic infinite moral value and worth
(3) Moral values and duties are properly basic.
Premise (2) presupposes God necessarily possesses intrinsic infinite moral value and worth (hence He is worthy of all worship) and created all mankind in His image with contingent intrinsic infinite moral value and worth to reflect God.
Moreover, God is that being which no greater can be conceived, thus He is the most perfect being. As the most perfect being, God is goodness itself; therefore, He is both the starting and stoping point of moral values and duties. God is necessarily both the one and many, object and subjects, particular and universals, descriptive and normative. God 'is' what He 'ought' to be. Since 'ought' is derived from God's commandments as they express His character, will and nature. He is both simultaneously the substance and the ideal. He possesses both the indicative and imperative. Therefore, God is the ground of all ontic, epistemic and moral norms.