Short answers, pulled from the story.
The 900 Theses were a set of propositions on religion, philosophy, natural philosophy, and magic that Pico published in December 1486 and proposed to defend in public debate in Rome. Pope Innocent VIII condemned them as partly heretical, scandalous, and favoring arts hostile to the Catholic faith; nearly all copies were burned, making this the first printed book to be universally banned by the Church.
The Oration on the Dignity of Man, written in 1486, was Pico's introduction to his 900 Theses. It argued that human vocation is a mystical one requiring moral transformation, intellectual inquiry, and union with absolute reality, and justified the human quest for knowledge by blending Neoplatonism and Aristotelian Scholasticism. Later generations called it the Manifesto of the Renaissance.
While recovering from injuries in Perugia, Pico encountered the mystical Hebrew Kabbalah through Rabbi Johannan Alemanno and received Kabbalistic texts, some of which were forged or doctored by the convert Flavius Mithridates. Persuaded that Kabbalistic mysteries were keys to heaven for Christians, Pico incorporated the tradition into his 900 Theses, making him the acknowledged founder of Christian Kabbalah.
Pico died on the 17th of November 1494 at the age of 31 under mysterious circumstances, on the same day Charles VIII of France entered Florence. In 2007, his body was exhumed from the Church of San Marco in Florence, and forensic testing showed he likely died of arsenic poisoning, possibly at the order of Lorenzo de' Medici's successor, Piero de' Medici.
Pico met Girolamo Savonarola during a visit to Florence as a young student and remained a close friend for life. Pico personally persuaded Lorenzo de' Medici to invite Savonarola to Florence. Near the end of his life, Pico became a follower of Savonarola, destroyed his own poetry, gave away his fortune, and resolved to become a monk.
The beardless young man in Raphael's fresco The School of Athens, painted between 1509 and 1511, is thought by some scholars to be Pico della Mirandola. Art historian Christiane Joost-Gaugier identified Pico as a major philosophical inspiration for the fresco's program, especially for its portrayal of the harmony between Plato and Aristotle.