Claims that this motto is now “traditional” are irrelevant. Slavery was abolished in the United States even though it had been traditional for 300 years in the New World. Just because something is traditional doesn’t mean that it’s right. Claims that this motto is patriotic or nationalistic are empty. The motto mentions no nation by name. Claims that this motto is non-religious, is part of a secular “civic religion” or is in some other way innocuous are ludicrous. Whatever else it may be, it is a statement that promotes the idea that religious belief is a requirement of citizenship. That goes directly against the First Amendment.

Regardless of whatever else it may be, the phrase “In God We Trust” is, on its face, inherently a statement of religious belief. One cannot trust in a god unless one first decides to believe, on faith, that some such thing actually exists. That reliance on belief through faith makes any statement of trust in a god a statement of religious belief.
“It has been noted that the prohibition against governmental endorsement of religion ‘preclude(s) government from conveying or attempting to convey a message that religion or a particular religious belief is favored or preferred.’ Moreover, the term ‘endorsement’ is closely linked to the term ‘promotion,’ and this Court long since has held that government ‘may not […] promote one religion or religious theory against another or even against the militant opposite.’ The Establishment Clause, at the very least, prohibits government from appearing to take a position on questions of religious belief or from ‘making adherence to a religion relevant in any way to a person’s standing in the political community.’”

Supreme Court majority opinion, County of Allegheny v. American Civil Liberties Union, 492 U.S. 573 (1989)

“…any endorsement of religion (is) invalid, because it sends a message to nonadherents that they are outsiders, not full members of the political community, and an accompanying message to adherents that they are insiders, favored members of the political community.”

Supreme Court Justice Sandra Day O’Connor, Lynch v. Donnelly, 465 U.S. 668 (1984)
“The ‘establishment of religion clause’ of the First Amendment means at least this: Neither a state nor the Federal Government can […] pass laws which aid one religion, aid all religions, or prefer one religion over another.”
Supreme Court Justice Hugo Black, Everson v. Board of Education, 330 U.S. 1 (1947)

“The day that this country ceases to be free for irreligion, it will cease to be free for religion – except for the sect that can win political power.”

Supreme Court Justice Robert H. Jackson, Zorach v. Clauson, 343 U.S. 306 (1952)