Leonardo Da Vinci, born in the village of Vinci in Tuscany, Italy, on April 15, 1452, was one of the most important figures of the Renaissance. While people may realize that he as an important artist, though, they don't generally realize how important he was as an early skeptic, naturalist, materialist, and scientist.
There is no evidence that Leonardo Da Vinci was an atheist, but he was an early a role model in how to approach both scientific and artistic problems from a naturalistic, skeptical perspective. Modern atheistic humanism owes a great deal to Renaissance Humanism generally as well as many individual Renaissance humanists like Leonardo.
Art, Nature, and Naturalism
Leonardo Da Vinci believed that a good artist must be a good scientist to best understand and describe nature. This was what made the Renaissance Man which Leonardo was such a good example of: belief that integrated knowledge of diverse subjects made a person better in all those individual subjects. This was also why Leonardo was such a strong skeptic, casting doubt on many of the popular pseudosciences of his day - especially astrology, for example.
One reason why Renaissance Humanism was a major break from Medieval Christianity was the shift in focus away from faith and otherworldly concerns and towards empirical investigations, naturalistic explanations, and skeptical attitudes. None of this was pursued enough to establish a secular, atheistic alternative to theistic religion, but it laid the groundwork for modern science, modern skepticism, and modern freethought.
Skepticism vs. Gullibility
This is why the real Leonardo Da Vinci was so unlike Dan Brown's book. The Da Vinci Code doesn't encourage the intellectual values of skepticism and critical thinking which Leonardo himself both championed and exemplified (even if imperfectly). Dan Brown's book is instead founded upon a massive conspiracy of political and religious authorities and secrets. Dan Brown in effect encourages replacing one set of religious myths with a different one based on faith in the power of conspiracies.
Moreover, the very title of Dan Brown's book The Da Vinci Code actually means The from Vinci Code because "Da Vinci" is a reference to Leonardo's town of origin, not his surname. This is perhaps a relatively minor error, but it's representative of Brown's failure to pay attention to historical details in a book that purports to be based on historical truth.