In 1955 the US Congress passed a bill requiring that “In God We Trust” appear on all American currency.

Midst the Cold War threat of atheistic communism, the bill passed with bipartisan support and was signed into law by President Eisenhower.

Since all coins already complied with the law, the only changes were made to the paper currency.

In 1956, Congress passed a joint resolution declaring IN GOD WE TRUST the national motto of the United States.

The resolution, passed by both the House and the Senate unanimously without debate, replaced E pluribus unum, which had existed before as a de facto official motto of the United States of America.

Main Source: Wikipedia