"Modernists" are people within Christianity who seek to turn their church or denomination away from Christ. In the early twentieth century, they were opposed by the "fundamentalists," who believed the fundamental Christian doctrines. The fundamentalists were not a new group; Christianity had always believed in the Deity of Christ and His virgin birth, the literal Second Coming of Christ, His literal resurrection from the dead, salvation by faith alone, and the total innerrancy of the Bible.
Having rejected Christ, modernists also rejected holy living, which is a natural product of the new birth (which they also rejected). Fundamentalists over-reacted, preaching holy living (which the Bible commands) but adding man-made rules that dealt with the current situation.
They preached against movies, cards, women wearing slacks, and various other offenses. At one time, some of these rules might have made some sense. But as the decades rolled by, the "current situation" that had produced these rules changed. Fundamentalists found themselves stuck with obsolete rules that their own people did not want to obey. Their response? Preach heavily on these rules; it was the only way to keep them in force.