Whose unexpected reaction after one of george whitefield's sermons demonstrated how powerful whitefield's preaching style was?

The answer is Benjamin Franklin.

He was the first to turn his back when Whitefield started his sermon. However, because of the number of people in the venue of Whitefield's sermon was very crowded. The popularity of Whitefield allowed him to think differently. Because of this he offered to print the sermons of the preachers so people could buy them. He also provided a home for the preacher and his minister.

i confess that there are several parts of this constitution which i do not at present approve, but i am not sure i shall never approve them: for having lived long, i have experienced many instances of being obliged by better information or fuller consideration, to change opinions even on important subjects, which i once thought right, but found to be otherwise. it is therefore that the older i grow, the more apt i am to doubt my own judgment, and to pay more respect to the judgment of others.