Complex sentence: His original efforts were to attack those who criticized the Bible, but by the 1920s he added frontal attacks on evolution, which saw as "a principal cause of unbelief in the Bible and the gospel message it conveyed" (Davis, 1995).
Split into simpler sentences: His original efforts were to attack those who criticized the Bible, but by the 1920s he had changed to attacks on evolution. He saw evolution as "a principal cause of unbelief in the Bible and the gospel message it conveyed" (Davis, 1995).