Thomas Jefferson used the phrase "a wall of separation between Church and State" to explain that religion and government should stay separate from each other. He thought it was important to keep these two things apart so that everyone could have the freedom to practice their own beliefs without the government telling them what to do.
Jefferson believed that if the government got too involved with religion, it could lead to unfair treatment or make people feel forced to follow certain beliefs. By keeping them separate, people could believe what they wanted and the government could treat everyone equally, no matter their religion. This way, everyone could live freely and peacefully.