“By 2050, less than 20% of the United States will attend church on a regular basis (at least 3 times a month)”

http://www.futuretimeline.net/23rdcentury/images/christianity-trend-usa.htm

However it is not a linear trend and with the facilitation of information exchange, irreligiosity will accelerate as it has in secular countries in western europe.

In fact, In Spain, importance of religion has dropped by 20% between 1980 and today, and Spain is considered a religious country.