Although “In God We Trust” is the official motto, “E Pluribus Unum” has long been acknowledged as a de facto national motto. After all, it is on the Great Seal of the United States, which was adopted in 1782. In July 1776, almost immediately after signing the Declaration of Independence, John Adams, Benjamin Franklin, and Thomas Jefferson were tasked with designing a seal and motto for the new nation. Only the motto “E Pluribus Unum” (“from many, one”) survived from the committee on which Adams, Jefferson, and Franklin had served. All had agreed on that motto from the beginning. The current motto, “In God We Trust,” was developed by a later generation. It was used on some coinage at the height of religious fervor during the upheaval of the Civil War. It was made the official national motto in 1956, at the height of the Cold War, to signal opposition to the feared secularizing ideology of communism.

Source: Origins