You can indeed infer the existence of God simply by observing creation. Aristotle and Plato did it.
Does that tell you everything about God and what he requires? No. Revelation is required for that. But revelation can also be subjected to historical observations and be determined to be trustworthy and therefore reliable.
Is faith also required? Yes, but not at the expense of reason. A certain level of faith is required for almost any body of knowledge we acquire on the authority of others. That doesn't mean that particular body of knowledge is subjective.
No you cannot infer god based on nature alone. That is why the group of the scientists is one of the most atheists you can find. Science does not point to the need of the supernatural.
Most modern scientists are atheists because their philosophy is bad and they carry their poor philosophical presuppositions over into their science. Most scientists throughout history have been theists.
No, that is not the reason. The reason is because they know a lot about the natural world, way more than any theoretical philosopher.