Short answers, pulled from the story.
George Holyoake spoke the word secularism for the first time in 1851 before a crowd in London. He chose this specific term because he found the existing label of atheism too aggressive and divisive for his audience.
France enacted a strict separation law known as laïcité in 1905 that legally removed all religious influence from government operations. This system required the state to remain officially distanced from all religions and non-religious philosophical convictions in every official dealing.
Turkey adopted a similar model under Mustafa Kemal Atatürk starting in 1923 when he declared laiklik as the state ideology. Atatürk aimed to modernize the country by replacing Ottoman religious traditions with Western-style secular governance.
Communist regimes often implemented state atheism as scientific atheism to eliminate what they viewed as superstition. Marxism provided the ideological foundation for these systems that sought to replace traditional religion with materialist philosophy.
John Rawls published A Theory of Justice in 1971 which introduced the concept of overlapping consensus for political philosophy. His work argued that secular arguments should be treated the same way as religious ones within democratic discourse.