"the average American can get into the kingdom of heaen much more easily than he can get into the Boulevard St. Germain"

This recalls a Biblical quotation: It is easier for a camel to go through the eye of a needle than for a rich person to enter the kingdom of God. (Mark 10:25)

Elliott's point is to emaphasize how prestigious and select is this segment of Parisian society. The Boulevard St. Germain on the Left Bank represented the old aristocracy, as opposed to the new bourgeoisie of the Right Bank.