From the Catechism of the Catholic Church (who better to know about sin?):

Submission to authority and co-responsibility for the common good make it morally obligatory to pay taxes, to exercise the right to vote, and to defend one's country. (Paragraph 2240)

It goes on:

2242 The citizen is obliged in conscience not to follow the directives of civil authorities when they are contrary to the demands of the moral order, to the fundamental rights of persons or the teachings of the Gospel. Refusing obedience to civil authorities, when their demands are contrary to those of an upright conscience, finds its justification in the distinction between serving God and serving the political community. "Render therefore to Caesar the things that are Caesar's, and to God the things that are God's."   

2243 Armed resistance to oppression by political authority is not legitimate, unless all the following conditions are met: 1) there is certain, grave, and prolonged violation of fundamental rights; 2) all other means of redress have been exhausted; 3) such resistance will not provoke worse disorders; 4) there is well-founded hope of success; and 5) it is impossible reasonably to foresee any better solution.

Ain't this inneresting???