Complex sentence: The book was a surprisingly big seller for a conservative religious writer's first book and turned Steichen, who had been writing for a long time in small Catholic journals, into a significant figure in the move to restore a much tighter orthodoxy within the Church.
Split into simpler sentences: The book was a surprisingly successful for a conservative religious writer's first book. It turned Steichen, who had been writing for a long time in small Catholic journals, into a significant figure in the move to restore orthodoxy within the Church.