A popular refrain in philosophy is that you can’t get an ought from an is. However, oughts are fundamentally shaped by what is. Ethics is ultimately determined by metaphysics and epistemology (metaphysics are our fundamental assumptions about what is and epistemology is how you know it). Metaphysics and epistemology are the most antecedent assumptions that one makes that causally lead up to a person’s ethics.
For example, the Christian religion holds many metaphysical ideas that ultimately shape its ethics. Christianity’s belief in God, like many monotheistic religions, is central to its worldview. There are certain metaphysical assumptions that come along with the Christian god. For example, a Christian believes that God has direct access to everyone’s mind and that he knows everything in existence (God is omniscient). This knowledge is used to ultimately punish or reward people after they die (they go to either heaven or hell). He created this world as a test to see if a person would hold faith in Him (the fundamental Christian ethical test). Because of a Christian’s metaphysical and epistemological beliefs, they will guard their thoughts and actions for God’s sake.
Before someone can give you an ethical system, they must first develop a metaphysical and epistemological system, so before I can tell you what to do there must first be a “what.” This “what” forms what a person is ultimately, ethically supposed to do by describing the world that people act in. Without the “what” there is no subject nor objects to take part in an ethical action or to take ethical action upon. For Christianity, the existence of a god with edicts creates an imperative to please that god. For an atheist, the non-existence of a god means there isn’t an imperative to please a god. Ethical actions are ultimately determined by what IS, and metaphysics is the most fundamental understanding of what is. Without a belief in God, there is no ethical imperative to please him.
Ethics is a science that requires certain entities in order for it to be meaningful. Metaphysics and epistemology provide the entities necessary for ethics. Without an understanding of what “is” one cannot provide an understanding of what “should be,” for you wouldn’t have an understanding of what world that which “should be” would be in. Only with the establishment of a metaphysics and epistemology does one have an ethics.