Most players are aware that there was a time in his life when Fischer, like Reshevsky, would not play on the Sabbath (sundown Friday until sundown Saturday) but it had nothing to do with Fischer having been born Jewish. It had to do with his membership in the Worldwide Church of God.
First, some background on the church:
The church was founded by Herbert W. Armstrong (31 July 1892 - 16 January 1986) in the late 1930s. He also founded Ambassador College with campuses in California and Texas. Armstrong was an early pioneer of radio and televangelism. His teachings eventually came to be known as Armstongism and they included the interpretation of Biblical prophecy in light of something called British Israelism. This doctrine teaches that people of Western Europe, especially those of Great Britain, are direct descendants of the so called Ten Lost Tribes of Israel. In addition he taught observance of parts of the Law including dietary prohibitions, and the covenant law, Holy Days as well as observing the Sabbath from sundown Friday until sundown Saturday. He also taught many other ideas that were rejected by orthodox Christianity.
Armstrong founded the Ambassador International Cultural Foundation which promoted the arts, humanities, and various humanitarian projects. It was in this role that he met with heads of governments in various nations.
Armstrong taught, and his followers believed, he was Jesus’ first apostle since the first century and that God only works through one man at a time and that he was God's man for his time.
Eventually Herbert turned the organization over to his son, Garner Ted Armstrong (February 9, 1930 – September 15, 2003). Noted for his charisma, movie star looks, and for being a music enthusiast, he toyed with becoming a nightclub singer before following his father into the ministry. In radio and TV programs he mixed political, economic, and social news of the day with Bible-based commentary. Armstrong's voice, style and presentation attracted millions. Eventually Garner Ted got involved with disagreements with church leadership and his father and was involved in some sexual scandals and was disfellowshipped by his father in 1978.
In 2009, the Worldwide Church of God officially changed its name to Grace Communion International, underwent major changes in doctrine, rejected the Armstrong’s teaching and claim to have embraced orthodox Christianity.
During the time of the Armstrong’s great popularity their appeal to a lot of people was the free literature they offered. Literature covered their ideas on prophecy and salvation, but the big attraction was their advice on how to handle things close to everyone’s heart: finances, marriage and family. When a convert joined the church they discovered Armstrong’s teaching concerning tithing of one’s income. He claimed the Bible taught paying three tithes. If you can imagine paying, say, 20 percent of your income in taxes, then paying 30 percent (of the gross) to the church, you can imagine the hardship this would cause, but people were told that if they didn’t pay or left the church they would go to hell and it was this fear that held a lot of them.
This brings us to Fischer. He followed the church in the late 1960's to its headquarters in Pasadena and remained there despite his eventual break with the church. After winning the World Championship he “tithed,” measured by today’s purchasing power, nearly a half a million dollars.