Any state that makes a license plate with the words of IN GOD WE TRUST or any plate like that is a volation of the states and usa federal government laws of the separation of church and state laws against promoting god and reglion.
The pertinent part of the 1st amendment that applies to religious practices is "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof"...
The Establishment Clause of the First Amendment prohibits the establishment of a national religion by the Congress or the preference of one religion over another, non-religion over religion, or religion over non-religion. Originally, the First Amendment only applied to the federal government. Subsequently, under the incorporation doctrine, certain selected provisions were applied to states. It was not, however, until the middle and later years of the twentieth century that the Supreme Court began to interpret the Establishment and Free Exercise Clauses in such a manner as to restrict the promotion of religion by state governments. For example, in the Board of Education of Kiryas Joel Village School District v. Grumet, 512 U.S. 687 (1994), Justice David Souter, writing for the majority, concluded that "government should not prefer one religion to another, or religion to irreligion".
The Free Exercise Clause allows that any PERSON may practice his or her religion openly without question.
The "In God We Trust" slogan on any license plate is perfectly legal if:
1). - Having the statement on the particular plate that an individual uses is totally under control of the bearer of the plate. The use of the slogan is a decision that is made only be the plate user.
2). - The state can have no input into using the slogan. The state cannot give any preference to using the slogan on a plate, pro or con.
Also the term "God" is not necessarily Christian. The term "God", "Allah" or any other word referring to a supreme being can be used and it does not signify or promote any particular religion over any other religion.