"The government of the United States is not, in any sense, founded on the Christian religion." - John Adams, 2nd president of the United States

Never codified by law, E pluribus unum was considered a de facto motto of the United States until 1956 when the United States Congress passed an act (H.J. Resolution 396), adopting In God We Trust as the official motto. [...]
Originally suggesting that out of many colonies or states emerge a single nation, [E Plurbus Unum] has come to suggest that out of many peoples, races, religions and ancestries has emerged a single people and nation.
Wikipedia | E Plurbus Unum

A law passed by the 84th Congress (P.L. 84-140) and approved by the President on July 30, 1956, the President approved a Joint Resolution of the 84th Congress, declaring IN GOD WE TRUST the national motto of the United States. IN GOD WE TRUST was first used on paper money in 1957, when it appeared on the one-dollar silver certificate. The first paper currency bearing the motto entered circulation on October 1, 1957.
U.S. Treasury | Fact Sheet

The same Congress had required, in the previous year, that the words appear on all currency, as a Cold War measure: "In these days when imperialistic and materialistic Communism seeks to attack and destroy freedom, it is proper" to "remind all of us of this self-evident truth" that "as long as this country trusts in God, it will prevail."
Wikipedia | In God We Trust