a little gadget on the Percept website where you can type in a Zip code and immediately find out what percent of the people living there report "No Faith Involvement." For example, in my little corner of God's Country here in the upper Midwest, we manage to beat the national average (35%) by an exemplary three points, coming in at 32%!
Census data puts my Zip code at about 45,000 in the year 2000. That means there are around 14,000 people with no faith involvement essentially in my backyard.
As they say, North America is a mission field.