From The Economist I find that “rates of interfaith and inter-denominational marriage are rising, to the point where 45% of marriages in the past decade have involved either two religions or Christian doctrines that clash seriously.” The article observes that “Many are models of tolerance and creativity.”
Americans are more likely to marry someone of a different faith than someone who supports a different political party. Jews spotted the trend early, with a survey triggering alarm in 1990 when it claimed that more than half of American Jews were marrying out. Interfaith marriage remains most frequent among Jews (and rarest among Mormons, of all creeds studied). But others have been catching up, with overall rates of such unions more than doubling since the 1960s.
The Economist seems to suggest that greater tolerance is at work here and it accounts for this trend of interfaith marriage. My cynical side on this Monday morning is skeptical and I suspect, rather, that this data indicates how insignificant religious belief is for North Americans, and how privatized faith has become, such that it has little effect on how people live their public lives.