"Fewer than half of American adults (45%) now completely agree with the statement that "prayer is an important part of my daily life;" that number has declined from a high of 55% in 1999, although adding in those who mostly agree with the statement, fully 78% of Americans endorse the importance of prayer. Large numbers also agree completely or mostly that "we will all be called before God at the Judgment Day to answer for our sins" (79% down from 84% in 1994) and also at least mostly agree that they never doubt the existence of God (83% down from 88% in 1999)."
PewResearch 14. May 2007
Check the links for graphs.