The Pledge of Allegiance was originally written in 1892 by Francis Bellamy in honor of the 400th anniversary of Columbus's discovery of America. It was officially adopted by Congress in 1942, with the name "Pledge of Allegiance" being deemed to it in 1945.
The phrase "under God" wasn't originally in the Pledge of Allegiance. It was added in 1954 under the direction of President Eisenhower. Eisenhower fought for the addition after attending a service at the New York Avenue Presbyterian Church for Lincoln Sunday. There he heard Rev. George Docherty give a sermon advocating for the addition of "under God" to the Pledge of Allegiance. Doherty, originally from Scotland, came over to America in 1950 and was confused that Americans didn't have any clear connection to God. He compared this experience to the United Kingdom, where it's customary to say "God save our gracious King/Queen". Noting that Americans had no such phrase to link their country to God, he thought it necessary to add the phrase "under God" in the Pledge of Allegiance. Docherty also believed that the phrase "under God" was applicable to other religions as well, such as Judaism and Islam, since they too believed in a higher power. He specifically noted, however, that it doesn't apply to atheists. He believed atheists were a contradiction to the very ideals America was founded on (rights for every man given by a Creator). While the phrase could be taken to mean God in an ambiguous way, not being connected to a certain religion, the very proposition for the addition came from a pastor. Docherty's argument from a Christian perspective is what convinced Eisenhower to sign off on the addition, thus inescapably placing favoritism on Christianity.
Just as Rev. Docherty said, the addition of "under God" can be applicable for religions other than Christianity. It doesn't matter why it was added to the Pledge of Allegiance, it matters that it's applicable for several religions.
[P1] The Pledge of Allegiance was adopted in 1942 with no mention of God in the original document. [P2] The phrase "under God" was added in 1954 and was intended to tie the country to God and Christianity. [P3] It unfairly favors Christianity over other religions and therefore should be removed.
Rejecting the premises
[Rejecting P3] Other religions (Judaism and Islam for example) believe in a God, so the addition of the phrase doesn't discriminate against those religions.