The following context comes from the book "Black Rednecks And White Liberals" by Thomas Sowell:
By contrast, religious services in colonial Massachusetts developed what has been called the “meeting and lecture” approach, where the “style of preaching was a relentless cultivation of the plain style.” these “addresses tended to be closely argued statements of great density, in which Puritans reasoned as relentlessly with their maker as they did with one another." This intellectual approach to religion carried over into their daily lives.
What is the meaning of this sentence? Especially the phrase plain style. What does it mean to cultivate the plain style. (PS. I know what cultivate means so you don't have to explain that)