The problem is that once you blend Christianity and the State, you lose the virtues of Christianity while gaining the corruption inherent in politics. The Church is in contrast to government.
It is voluntary, not coercive, although in the past, governments have tried to control the church while mandating membership and tithing. We certainly don't want another Church of England like we had in colonial times.
It not bound by geography, unlike governments which make territorial claims for their nations, and declare foreigners unclean. The Gospel has always been for everyone willing to accept it, regardless of skin color nor nationality.
The church is designed for service to the people. Governments are always designed to benefit the political class by plundering the productive economy through taxes, price controls, etc. while telling us the table scraps from their stolen feast is mana from heaven.
This is why nationalism and christianity cannot combine. "The words 'Christian state' resemble the words 'hot ice'. Such an entity is either not a state, or it is not Christian." - Leo Tolstoy (specific source document currently unknown)