Again I remind you of the context in which Romans was written, when there was no such thing as a Christian government. Romans 13 must certainly have applied to all of its readers, including most certainly its first readers. So Romans 13 was not limited to Christian civil governance.
Certainly, there will be occasions when our mandate to obey God will conflict with our duty to obey the government; and in such cases God must certainly prevail. However, there are many functions and regulations of civil society that are not fully described, prescribed, or conradicted by Scripture, and in such cases we are to respect the laws of the land.
From the article:
The following five major theological problems are created when Christians tell our Magistrates that they need pay no heed to the specific Authority granted them in Romans 13: 8 - 10: