Who decided to put 'under god' in the Pledge of Allegiance?

Short answer: Christians.

Louis A. Bowman, an attorney from Illinois, was the first to initiate the addition of "under God" to the Pledge.

In 1951, the Knights of Columbus, the world's largest Catholic fraternal organization, began including the Words "under God" in the Pledge of Allegiance. Over the next two years, the idea spread throughout Knights of Columbus organizations.This campaign led to several official attempts to prompt Congress to adopt the Knights of Columbus' policy for the entire nation.

Prior to February 1954, no endeavor to get the Pledge officially amended succeeded. The final successful push came from pastor, George MacPherson Docherty. On February 7, 1954, with President Eisenhower attending, he delivered a sermon based about what he felt was missing from the Pledge. He cited Lincoln's Words "under God" as defining Words that set the United States apart from other nations.

President Eisenhower had been baptized a Presbyterian, just a year before. He responded enthusiastically to Docherty in a conversation following the service. Eisenhower acted on his suggestion the next day and on February 8, 1954, Rep. Charles Oakman (R-Mich.), introduced a bill to that effect. Congress passed the necessary legislation and Eisenhower signed the bill into law on Flag Day, June 14, 1954.

en.wikipedia.org/wiki/Pledge_of_Allegiance#Addition_of_.22under_God.22