A Californian father told America's highest court last week that the phrase "under God" in the Pledge of Allegiance, recited daily in most US schools, violated his daughter's religious freedom and amounted to government-sanctioned indoctrination.
Arguing that the religious reference contravenes the US constitution, Michael Newdow is suing Elk Grove Unified school district in Sacramento where his nine-year-old attends school.
But the Supreme Court's ruling, expected in July, will determine the school ritual's fate across America, where the teacher-led oath is compulsory in 43 states.
"Every morning, teachers have their students stand up, including my daughter, face the (US) flag, place their hands on their hearts, and affirm... ours is a nation under some particular religious entity," he told America's most senior judge during the hearing.
"I don't believe in God. And every morning my child is asked to say her father is wrong."
Judges said his daughter was free to opt out, but Mr Newdow said peer pressure placed an unfair burden on children.
"Imagine you're the one atheist with 30 Christians," said Mr Newdow, who has turned atheism into a religion as an ordained minister of the First Amendment Church of True Science.
Religion in schools is highly charged in a country whose founding charter explicitly demarcates church and state, but where tens of millions, including the president, espouse fundamentalist Christian beliefs. The Supreme Court outlawed mandatory prayer in US schools in 1962, but the issue has since come before it seven times.
The judges weighed in on the pledge, typically recited before lessons commence, after a San Francisco court sided with Mr Newdow in 2002, outlawing it across nine states. That ruling was condemned by President Bush and suspended pending a Supreme Court decision.
In a measure of the passions aroused, school chiefs in Madison, Wisconsin, triggered an "immediate, humungous, nationwide reaction" in 2001, spokesman Ken Syke recalled, after replacing the pledge with an instrumental version of the US national anthem played over school loudspeakers. More than 100 people packed a hearing that ran until 2am, reinstating the pledge, he said.
Mr Newdow, who has received death threats, faces a further challenge from his daughter's mother, who says the child does not mind saying the pledge.
Their custody battle annuls Mr Newdow's right to bring the case on his daughter's behalf, say opponents.