I understood that the founding fathers wanted to create a secular state and did not want to create the USA as a christian nation. Recently in a class I was bombarded by students saying "Under God" is in the constitution; while I highly doubt it I wanted to corroborate with the experts. Please I will ask for you to leave a reference as your answer as this information will need evidence.
This site has an excellent discussion about how the word "God" does NOT appear in the U.S. Constitution. By implication "Under God" does not appear there either, so your friends are wrong. The discussion in this site goes further to discuss how the absence of the term "God" cannot be meant to imply that the Founding Fathers were secularists along the lines of what that term means today; that concept is a recent one, and although those who developed and influenced the text of the constitution wished to avoid entangling religion and government, they did not have the more radical point of view of modern secularists.
This additional site also has a good discussion about that fact that the term "God" does NOT appear in the constitution.