When did they start putting In God We Trust on US paper money?

The motto reads "In God We Trust". It should be noted that the motto was added to coinage and currency during troubled times when people needed to reinforce the belief that the God of The Bible is on our side.

The phrase was added to currency during the same decade "One Nation Under God" was added to the Pledge of Allegiance as a similar response to "godless communism".

Coins

The motto was first put on 2¢ pieces in 1864, during the Civil War, as an expression of faith in terrible times. It was gradually added to other denominations of coins over the years; the last denomination to get the motto was the nickel starting in 1938.

Bills

Many people are surprised to find older paper money without the motto, but in fact its use wasn't extended to bills until about a half-century ago. In response to the perceived threat of "godless communism" during the Cold War, in 1955 Congress ordered that the motto also be placed on paper money. The first bills to carry it were $1 silver certificates printed during and after 1957. These included a new 1957-dated series and some 1935-G and all 1935-H bills that were printed at the same time. By 1963 all other denominations had been redesigned to include the motto as well.

Rumors of Removal

Rumors persist among right-wing and fundamentalist religious groups that there is a "conspiracy" to remove the motto from American money. Those rumors were made worse when a few 2007-dated Presidential dollars were accidentally minted without the motto when mint employees forgot to add edge lettering to several batches of the new coins. Despite the fact that the mistake also deleted the coins' date, mint mark, and the motto "E Pluribus Unum" as well as "In God We Trust", the error was seized on as another example of a clandestine attempt to secularize American currency. In reality the presence of the motto is not an issue for the vast majority of Americans, and there are NO plans whatsoever to change it.