Why does the US dollar bill read In God We Trust?

"In god we trust" is a national motto of the United States and has to appear on all banknotes, by law, since 1955.

While the motto first appeared on 2¢ pieces in 1864 and had been on all coins since 1938, it was never printed on bills. Cold-war patriotism led Congress to authorize its addition to bills as well.

The first notes with the motto were Series 1957 and 1935G $1 silver certificates that entered circulation simultaneously in 1957. Over the next several years it was added to all other denominations.