Why it is important to separate religious the state?

In the United States the First Amendment to the Constitution states that there should be no establishment of religion--

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.

Government should not foist any religion on me, nor should it prevent me to worship in the way I want.

Religion and government must be separate because our society is multi-faceted with numerous religious denominations and every other degree of belief and non-belief in deities. A government-sponsored religion would choose one and then force the people to obey the doctrines of that religion. Its doctrines would inform the laws, its religious leaders will gain too much power and that type of power corrupts. Religious majorities have historically been extremely unfair to minorities. The separation between church and state must be guarded in order to preserve freedom.

Most of the founders viewed religious interference as damaging to government. Most were deists who believe in the existence of a remote deity who got things started and then backed off, not interfering with the day-to-day existence of humankind. The type of religionists who want to deny church-state separation today seek to impose their man-made religious dogma on the society-at-large. That is exactly what the founders sought to avoid.