Final answer:
The U.S. Constitution's First Amendment, specifically the Establishment Clause, supports the notion of opposition to government-supported religion, ensuring no law can establish an official religion and no religious tests can be required for public office.
Step-by-step explanation:
Those who are opposed to the concept of government-supported religion would find support for this notion in the U.S. Constitution's First Amendment, specifically within the Establishment Clause.
This clause states that Congress shall make no law respecting an establishment of religion, which has been interpreted to mean that the government cannot establish an official religion or prefer one religion over another. Additionally, Article VI of the Constitution forbids religious tests for public office, ensuring that government representatives are not chosen based on their religious affiliations or beliefs.
The First Amendment ensures the separation of church and state, creating a buffer against government interference in religious matters, thus supporting the view that government should not back any single religion.