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.