Short answers, pulled from the story.
Pantheism is the belief that reality and divinity are identical: the universe, nature, or the totality of existence is itself God, with no separate creator required. The mid-18th-century English theologian Daniel Waterland defined it as the view that God and the whole universe are one and the same substance, one universal being, such that human souls are modifications of the divine substance.
The English mathematician Joseph Raphson introduced the Latin term "pantheismus" in his 1697 book De Spatio Reali seu Ente Infinito. The term was first translated into English in 1702, and popularized in English by Irish writer John Toland in his 1705 work Socinianism Truly Stated, by a Pantheist.
Baruch Spinoza, a 17th-century Dutch philosopher of Portuguese descent, is regarded as pantheism's most celebrated advocate. His posthumously published Ethics argued that God and the universe are one substance, opposing Rene Descartes' mind-body dualism, and became the primary source from which Western pantheism spread. He was excluded from his Amsterdam synagogue at age 23, and his Ethics was placed in the Catholic Church's Index of Forbidden Books.
Giordano Bruno, an Italian friar who preached about a transcendent and infinite God, was burned at the stake by the Roman Inquisition in 1600 for his pantheist views. He has since been recognized as a celebrated pantheist and martyr of science.
According to a letter written by Lincoln's law partner William Herndon in 1886, Lincoln was at one time "an elevated Pantheist" who believed the soul lost its individual identity but survived as a force. That letter sold at auction for US $30,000 in 2011. Herndon noted that Lincoln later moved toward a belief in a personal God.
Self-identified pantheists remain a small but measurable population. The 2011 censuses found the United Kingdom had the most pantheists of any counted country. Canada recorded 1,855 pantheists in 2021, representing 0.005% of the population. Ireland counted 1,940 pantheists in 2011, up from just 202 in 1991.