The First Amendment (1791) of the US Constitution reads, in full:
"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances."
1791 US Constitution
The portion of the First Amendment relevant to the debate over the inclusion of the words "under God" in the Pledge is often broken down by legal scholars into two clauses called "The Establishment Clause" and the "The Free Exercise Clause."
PRO "Under God" Interpretation
CON "Under God" Interpretation
"Congress shall make no law respecting an establishment of religion..."
Congress is permitted to add the words "under God" to government materials because the phrase does not "establish" any specific religion.
Congress cannot add any reference to "God" or religion to any government materials because this clause explicitly forbids any such inclusion.
Free Exercise Clause
"...or prohibiting the free exercise thereof..."
Americans are permitted to Pledge to a nation under any "God" they choose, or to not use any such words at all.
Congress is prohibited from adding the words "under God" because those words may interfere with the free exercise of atheism.