Consider two different worldviews.
Call the first one "the classical worldview". One major feature of it is final causality, or teleology. I'm thinking here of Aristotelian teleology, not of Paley-style "watchmaker" teleology. The difference is that the function or purpose of a thing is internal to that thing. According to Aristotelian teleology, a seed will just grow into a plant, given proper conditions, by its very nature. Whereas Paley treats living things as the parts of a watch, lying around until someone (God?) comes along and puts them together and forces them to behave like a watch, which they otherwise would not do.
Aristotelian teleology is associated with a theistic worldview. Plato had his vague proto-unmoved mover as well as Demiurge, and Aristotle had his unmoved mover. These were merged with Christian theology later, by Augustine and Aquinas.
So the worldview that includes Aristotelian teleology is tied up with some form of theism. It isn't strictly entailed, but there are no developed atheistic teleological worldviews. If one accepts teleology, then one implicitly accepts a theistic worldview.
The other worldview lacks such teleology. Call it "mechanistic philosophy". There are no functions, or final causes. Particles bop around blindly, occasionally colliding and clumping together into planets, people, etc. There are no functions or specific effects that anything is oriented toward, other than by chance.
Now consider any normative statement. Not even necessarily ethical norms, but even rational norms. "If the premises are true and the argument is logically valid, then one ought to accept the conclusion." Or "if one does not have enough evidence, one ought not believe it." These are all normative statements. But an "ought" implies that there is a purpose, or function. Human intelligence is oriented toward the function of discovering truth, which is why one ought to accept it when discovered. In short, an "ought" implies functions and final causality. And the lack of final causality implies that there are no "oughts", since nothing has a function or purpose.
1. If the classical worldview is false, then there are no rational norms
2. There are rational norms
3. Therefore, the classical (and hence theistic) worldview is true