The Knights of Columbus had apparently in 1951 instituted their own version of the Pledge of Allegiance for use at their meetings that contained the words "under God." Seeing that the time was right, they enlisted the cooperation of the American Legion in lobbying the Executive branch and the Congress to add "under God" to the pledge. Ignoring the Constitution and caving in to the expediency of the moment, President Eisenhower expressed support for the measure, and it was passed on Flag Day, June 14, 1954.
http://home.flash.net/~lbartley/au/issues/godtrust.htm
edit: and it wasnt until 1960 that the Catholics through Vatican II decided that when they become a majority in a nation that they would not force their religious fews on others through changing the government to a theocracy.