The word rapture means "extreme pleasure, happiness, or excitment." Ryan built his city to have discrimination or oppressive things in order to create a city of uninhibited artists who would feel VERY happy to do what they want freely. Iy was also populated by "taking away" people from the surface ("the Vanishment"), which is what the word rapture also alludes to.
P.S. Don't assume everyone has a belief in which there's such thing as a rapture.