In arguing against the separation of church and state, Christian Nationalists frequently promote all sorts of myths, misconceptions, and even lies about the relationship between religion and government. It's important to confuse people about how religion and government should interact because it helps the goal of convincing people that it's appropriate for the state to promote, endorse, or even fund one religion in particular. Seeing the right relationship between religion and government, however, reveals why the state should be secular and separated from religion.
Many opponents of church/state separation offer an argument which also seems to be a motivation behind their efforts to blend church and state: good government isn't possible without religion and religious values, so the introduction of religion into government is necessary. Few stop at just religion in general and argue Christian principles specifically are necessary for morality, civility
, and good citizenship. They deny that secular government can be good government.
Apologists for theocracy sometimes argue that not only is Secular Humanism a religion, but in fact that the Supreme Court has officially ruled that Secular Humanism is a religion. They then proceed to argue that Secular Humanism is taught in public schools in violation of church/state separation. Since they don't believe that church and state should be separated anyway, their entire point seems to be that public schools should indoctrinate kids into Christianity instead.
One tactic against church/state separation is to portray separation as anti-religious
. Because the American government cannot be hostile towards religion, it is argued that church/state separation is an illegitimate infringement on citizens' religious liberties. Their "solution" to what is really a non-existent problem is to mingle religion with government, thus promoting theocratic systems as if that could enhance religious liberty instead of, as is actually the case, undermine it.
Most arguments against church/state separation are secular — philosophical, political, historical, etc. Sometimes, though, opponents of church/state separation use theological and religious arguments. The purposing of making such an argument appears to be to convince people to reject the notion of separation on religious grounds — if it is not biblical, then it is not Christian, and if it is not Christian, then it is not something which a Christian should accept.