"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof..."
Layman's terms: The government cannot condone any particular religion, lest it restrict the religious freedom of its people. "Under God" was added to the Pledge of Allegiance in 1954. "In God We Trust" become this nation's de facto motto in 1956.
Now, is there anything wrong with believing these things? No. We have freedom of belief here. But by placing these things in our PLEDGE and on our MONEY, our government implicitly ordains a belief in God. That goes against the Bill of Rights and our basic American freedoms. It's the government saying "When you spend American money or pledge allegiance to America, you WILL believe and trust in God."
And that is not constitutional.