Question

Which prohibits the national government from supporting an official religion?

Answer 1

Is this in the US? I'd go with the constitutional right to freedom of religion. You could reference the Establishment Clause.

Answer 2

Unless you are talking about Iran or Rome no other countries outwardly support a certain religion take the U.S. and Turkey for example both could be classified as Christian or Muslim countries but don't lean towards a certain religion.

Hope I helped,

From a American Muslim. Peace!