He had a serious interest in alchemy
Newton spent much of his adult life, in addition to his scientific pursuits, pursuing another hobby, alchemy, with the objective of discovering the philosopher's stone, a material that reportedly could change ordinary metals like lead and iron into gold. He kept his alchemical experiments a secret and kept some of his findings in code.
Newton researched the Bible, among other things, in an attempt to uncover hidden messages about how the cosmos works.