"In God we trust " was adopted as the official motto of the United States in 1956 as an alternative or replacement to the unofficial motto of E pluribus unum, adopted when the Great Seal of the United States was created and adopted in 1782.
"In God we trust" has appeared on most U.S. coins since 1864 and on paper currency since 1957.
History
The phrase appears to have originated in "The Star-Spangled Banner", written during the War of 1812. The fourth stanza includes the phrase, "And this be our motto: 'In God is our Trust.'"
See further reading from above at: http://en.wikipedia.org/wiki/In_God_we_trust