Nature and nature's laws lay hid in night. God said "Let Newton be!" and all was light! Alexander Pope
Sir Isaac Newton was a very complex individual.
Not only did he revolutionize physics and mathematics, but he also made groundbreaking contributions in various other areas of science including astronomy and natural philosophy. His 1687 work Philosophiæ Naturalis Principia Mathematica is still considered one of the most influential books in the history of science. It's hardly surprising that his scientific contributions have made his name so well-recognized by scientists and the general public alike centuries after his death.
Perhaps not so well known were Newton's other accomplishments. His lifelong fascination with alchemy seemed to be a natural extension of his wish to uncover how all things in nature were interrelated. He was hardly unique in his alchemical research since many prominent intellectuals had been doing the same for centuries. Still, there was a certain irony in the fact that the scientific revolution that Newton ushered in helped make alchemy obsolete. Inspired by Elizabethan alchemist, John Dee as well as the Rosicrucians, Newton had one of the most extensive collection of alchemy texts in England. The full extent of his research into alchemy will likely never be known given that Newton went to great lengths to keep it hidden. His writings on the subject only surfaced long after his death and many of Newton's biographers tended to dismiss them as being a source of embarrassment for admirers of the great scientist.
In writing about alchemy, Newton stated that "They who search after the Philosophers' Stone [are] by their own rules obliged to a strict & religious life. That study [is] fruitful of experiments" which certainly fit in with his empirical search for understanding. There was also a touch of egoism at work since many of his contemporaries were also searching for the Philosopher's Stone (including Robert Boyle) and Newton didn't want one of them to discover it before him. Considering that much of the laboratory equipment developed by alchemists would later become part of the standard tools for chemists, Newton's research was more practical than it seems today. Suffice it to say, Newton never succeeded in finding the Philosopher's Stone but his lifelong fascination gave alchemists an added respectability that they might not have had otherwise. Newton's work in alchemy may also have played a role in his death in 1727. An autopsy on his body found elevated concentrations of mercury which may explain much of the eccentric behaviour he showed in life.
In his old age, Isaac Newton's interests turned in a new direction: theology. As with his work on alchemy, many of his theological writings were only published after his death. While he remained a devout Christian all his life, many of his ideas on religion were frequently at odds with the Church of England (and he might well have been considered a heretic in earlier times). He railed against purely mechanistic views of the universe and argued that God created the basic rules that governed nature. He also stressed that the Bible represented the source of all true wisdom and, towards the end of his life, Newton devoted himself to an in-depth examination of the books of Daniel and Revelation. Although never published in his lifetime (Newton was reluctant to antagonize the powerful Church of England),Newton's Observations on the Prophecies of Daniel and the Apocalypse of St. John was one of his most ambitious works, at least in terms of providing a reasonable chronology for the events described throughout the Bible. Drawing on other historical sources (such as Flavius Josephus) and using his own peerless mathematical skills, Newton carefully calculated, among other things, the Crucifixion as occurring on Friday, April 23, 34 CE.
In analyzing the prophetic texts, Newton carefully broke down each book by applying the same principles to Biblical study that he devoted to studying the heavens. Although some biographers dismissed his attempts as being a "rambling muddle", Newton used information from the book of Ezekiel and the writings of Maimonides to work out the dimensions of Solomon's Temple which he considered to be a key element in understanding Revelation. He also developed a future timeline based on his knowledge of astronomy and predicted the Second Coming of Christ in 1948 (although the Last Judgment wouldn't occur until around 3370 CE). Much of Newton's biblical work seemed to focus on hatred for the Catholic Church (which he considered to be the Antichrist) and the eventual coming of the "True Church" after the "Evil One" (Rome) was destroyed. Part of this hatred of Catholic doctrine stemmed from his opposition to the idea of the Trinity which he considered false. He also believed in a literal interpretation of Genesis by the way (although he did suggest that the seven days of creation may have been longer than twenty-four hours).
If his biographers were apologetic about Newton's fascination with alchemy, they were even less accepting of the theological writings that largely consumed the last years of his life. He always stayed true to the idea that the universe was inherently understandable and that theology, like science, was a reflection of God's meaning and plan for the human race. As the chief patriarch of the Royal Society (he was known as "Le Grand Newton" across Europe), Newton's scientific reputation was secure enough but he was still reluctant to publish his conclusions about the Bible. Weeks before his death, Newton burned a collection of papers at his house in Leicester Fields. While historians dispute whether the papers he burned were historically significant, one witness later suggested that Newton had burned several manuscripts as well. Considering the eccentric nature of the alchemical and theological works that did survive, it's hard not to wonder what else Newton was afraid would reflect poorly on him after his death.
Isaac Newton died on March 20, 1727 after a brief illness. After a public funeral, he was buried in Westminster Abbey in what is now known as "Scientist's corner" (which he shares with Charles Darwin and James Clerk Maxwell among others). A monument standing by one side of the Abbey shows Newton leaning on a pile of four books labeled "Divinity', 'Chronology', 'Optica' and 'Phil Princ. Math'". His more, unconventional, works aren't mentioned.