In the 1920s, Bryan shifted his focus away from politics, becoming one of the most prominent religious figures in the country.
He held a weekly Bible class in Miami and published several religiously themed books.
He was one of the first individuals to preach religious faith on the radio, reaching audiences across the country.
Bryan welcomed the proliferation of faiths other than Protestant Christianity, but he was deeply concerned by the rejection of Biblical literalism by many Protestants.
According to historian Ronald L. Numbers, Bryan was not nearly as much a fundamentalist as many modern-day creationists of the 21st century.
Instead he is more accurately described as a "day-age creationist".
Bradley J. Longfield posits Bryan was a "theologically conservative Social Gospeler".

In the final years of his life, Bryan became the unofficial leader of a movement that sought to prevent public schools from teaching Charles Darwin's theory of evolution.
Bryan had long expressed skepticism and concern regarding Darwin's theory; in his famous 1909 Chautauqua lecture, "The Prince of Peace", Bryan had warned that the theory of evolution could undermine the foundations of morality.
Bryan opposed Darwin's theory of evolution through natural selection for two reasons.
First, he believed that what he considered a materialistic account of the descent of man (and all life) through evolution was directly contrary to the Biblical creation account.
Second, he considered Darwinism as applied to society (social Darwinism) to be a great evil force in the world, promoting hatred and conflicts and inhibiting upward social and economic mobility of the poor and oppressed.

As part of his crusade against Darwinism, Bryan called for state and local laws banning public schools from teaching evolution.
He requested that lawmakers refrain from attaching a criminal penalty to the anti-evolution laws and also urged that educators be allowed to teach evolution as a "hypothesis" rather than as a fact.
Only five states, all in the South, responded to Bryan's call to bar the teaching of evolution in public schools.

Bryan was worried that the theory of evolution was gaining ground not only in the universities, but also within the church.
The developments of 19th century liberal theology, specifically higher criticism, had allowed many clergymen to be willing to embrace the theory of evolution and claim that it was not contradictory to Christianity.
Determined to put an end to this, Bryan, who had long served as a Presbyterian elder, decided to run for the position of Moderator of the General Assembly of the Presbyterian Church in the USA, which was at the time embroiled in the Fundamentalist–Modernist Controversy.
Bryan's main competition in the race was the Rev. Charles F. Wishart, president of the College of Wooster in Ohio, who had loudly endorsed the teaching of the theory of evolution in the college.
Bryan lost to Wishart by a vote of 451–427.
Bryan failed in gaining approval for a proposal to cut off funds to schools where the theory of evolution was taught.
Instead, the General Assembly announced disapproval of materialistic (as opposed to theistic) evolution.
In the 1920s, Bryan shifted his focus away from politics, becoming one of the most prominent religious figures in the country. He held a weekly Bible class in Miami and published several religiously themed books. He was one of the first individuals to preach religious faith on the radio, reaching audiences across the country. Bryan welcomed the proliferation of faiths other than Protestant Christianity, but he was deeply concerned by the rejection of Biblical literalism by many Protestants. According to historian Ronald L. Numbers, Bryan was not nearly as much a fundamentalist as many modern-day creationists of the 21st century. Instead he is more accurately described as a "day-age creationist". Bradley J. Longfield posits Bryan was a "theologically conservative Social Gospeler".
In the final years of his life, Bryan became the unofficial leader of a movement that sought to prevent public schools from teaching Charles Darwin's theory of evolution. Bryan had long expressed skepticism and concern regarding Darwin's theory; in his famous 1909 Chautauqua lecture, "The Prince of Peace", Bryan had warned that the theory of evolution could undermine the foundations of morality. Bryan opposed Darwin's theory of evolution through natural selection for two reasons. First, he believed that what he considered a materialistic account of the descent of man (and all life) through evolution was directly contrary to the Biblical creation account. Second, he considered Darwinism as applied to society (social Darwinism) to be a great evil force in the world, promoting hatred and conflicts and inhibiting upward social and economic mobility of the poor and oppressed.
As part of his crusade against Darwinism, Bryan called for state and local laws banning public schools from teaching evolution. He requested that lawmakers refrain from attaching a criminal penalty to the anti-evolution laws and also urged that educators be allowed to teach evolution as a "hypothesis" rather than as a fact. Only five states, all in the South, responded to Bryan's call to bar the teaching of evolution in public schools.
Bryan was worried that the theory of evolution was gaining ground not only in the universities, but also within the church. The developments of 19th century liberal theology, specifically higher criticism, had allowed many clergymen to be willing to embrace the theory of evolution and claim that it was not contradictory to Christianity. Determined to put an end to this, Bryan, who had long served as a Presbyterian elder, decided to run for the position of Moderator of the General Assembly of the Presbyterian Church in the USA, which was at the time embroiled in the Fundamentalist–Modernist Controversy. Bryan's main competition in the race was the Rev. Charles F. Wishart, president of the College of Wooster in Ohio, who had loudly endorsed the teaching of the theory of evolution in the college. Bryan lost to Wishart by a vote of 451–427. Bryan failed in gaining approval for a proposal to cut off funds to schools where the theory of evolution was taught. Instead, the General Assembly announced disapproval of materialistic (as opposed to theistic) evolution.
William Jennings Bryan