Religious Freedom
If the goal of our society is utilitarian happiness, then it's pretty obvious that allowing freedom of religion and worship would be a key component. Forcing anyone outside the mainstream to publicly deny one of the most important aspects of their identify is going to have some devastating consequences on the aggregate happiness and well-being of that society.
Separation of Church and State is a related political best practice that helps ensure that religious freedom is not eroded over time.
State Religion Undermines Faith
One of the core tenets of any religion is that the believer must come to the faith of their own free will. This principle is fully undermined when there is state sponsorship of religion. The coercive effects of even the mildest official endorsement, especially in the classroom where impressionable children see teachers as trusted authority figures.
While evangelists may find it tempting to use the power of government to endorse their belief system, any effort to do so undermines freedom of thought, leading to the faith of a rebellious child who goes through the motions to avoid getting in trouble but has no love for god, and would quickly leave if there were no consequences to doing so. Is it worth it just to boost attendance at services?
There are also the theological debates about which translation of scripture or sectarian interpretation is going to be held above all others, which inevitably results in the elevation of a minority option to an official position of the state. The vast majority of believers will have their freedom to worship the way they wish curtailed.