There is an old saying: “You should never discuss sex, politics, or religion in polite company.” The point of this saying is that it is much easier to have pleasant relationships when we avoid controversial topics. People generally have very strong opinions about sex, politics, and religion; and many of these opinions contradict each other. Today, I am not going to talk about sex or politics. Instead, I am going to talk about religion. I apologize beforehand if I offend you.
By religion, I mean more specifically that aspect of religion known as theology. Theology is the belief or beliefs that one has regarding God or The Gods. According to this definition, almost every human has a theology because almost every human has a belief regarding God. Many Jews believe that they are the Chosen People of God, most Christians believe that Jesus of Nazareth is the Only-begotten Son of God, most Muslims believe that Muhammad is the last and greatest prophet of God in human history, all Agnostics believe that mortal humans do not have enough intelligence and/or information to determine if God exists or not, and all Atheists believe that God does not exist.
Leonardo da Vinci (1452-1519) was arguably the most intelligent human who ever lived, so it makes sense that people would want to know what his theology was. Such people reason that such a smart person probably had a very good theology, maybe even the best theology, and such a theology could greatly help them. What we believe greatly influences how we think, feel, and act; and how we think, feel, and act greatly influence how sane, good, and happy we are in this life. What we believe might even greatly influence how sane, good, and happy we are in the next life, if there is such a thing.
What did Leonardo base his beliefs on? Was it the teachings of the Roman Catholic Church
into which he was baptized, was it the teachings of the Bible, or was it something else? Leonardo's writings indicate that he based all of his beliefs on reason. Reason is one's ability to perceive reality as honestly and completely as one can, and then make logical conclusions based on what one perceives. The opposite of reason is faith, and I like Mark Twain's definition of faith: “Faith is being convinced that what you don't believe is true.” More seriously, faith is believing something without sufficient reason. Reason leads us to conclude that human beings built the pyramids of Egypt by themselves. Faith might lead us to conclude that aliens helped the ancient Egyptians build them.
Marco Rosci's biography, Leonardo, notes that Leonardo "adopted an empirical approach to
every thought, opinion, and action and accepted no truth unless verified or verifiable, whether related to natural phenomena, human behavior, or social activities" http://www.straightdope.com/columns/read/1697/was-leonardo-da-vinci-religious). Leonardo himself wrote, “Anyone who conducts an argument by appealing to authority is not using his intelligence; he is just using his memory.”