In the Bill of Rights (adopted in 1791), the First Amendment grants freedom of religion.
If the phrase "under God" was to be removed from the Pledge of Allegiance, then that would result in a violation of religious freedom. Since the addition of "under God" was in reference to Christianity, removing the phrase would be discrimination against Christians, thus a violation of the First Amendment. In addition, "under God" could be in reference to many different religions. As there is a God figure in every religion. A God is "the being perfect in power, wisdom, and goodness who is worshiped (as in Judaism, Christianity, Islam, and Hinduism) as creator and ruler of the universe." Therefore it is discriminating against any religion that has a god or deity figure.
The phrase "under God" shouldn't have been added to the Pledge of Allegiance in the first place. It favors Christianity over other religions. For an individual who is an atheist, it is insulting to ask them to say the words "Under God" as they do not believe in the existence of a God-like figure. In addition, Jehovah's Witnesses though they do believe in a God do not believe in pledging to any symbol or object. Therefore, it is a violation of their religious freedom to force them to say those words. It is not a violation of religious freedom to leave out "under God", because people can still choose to say the phrase if they prefer. As much as people have the right to express their religion by saying "under God" in the Pledge of Allegiance, they also have the right not to say it as well.
[P1] The First Amendment ensures freedom of religion. [P2] The phrase "under God" is a referral to Christianity. [P3] Removing the phrase would discriminate against Christianity. [P4] Discriminating against a religion violates the First Amendment.
Rejecting the premises
[Rejecting P3] Allowing the phrase to stay discriminates against other religions.