Imagine, the nation you lived in came under the control of a President who was anti-clerical, anti-catholic, radically socialist, and militantly atheist. This President then started adding laws to your nation’s constitution which had the blatant purpose of making Catholicism Illegal. Priests no longer received civil liberties, religious organizations could no longer own land, and public worship was now illegal. Priests are already being deported, scared away, imprisoned, and outright murdered.
Many Catholics started fleeing in response to this, hoping to escape persecution by relocating to foreign lands. However, many decided not to take this sitting down, and intend to take back their country and overthrow this tyrant with force of arms.
This isn’t a hypothetical. The President is Plutarco Elias Calles, the nation is Mexico, the years were 1926-1929. Calles tried making Catholicism de-facto illegal in his nation, so in response thousands of Catholic Mexicans (called Cristeros) rose up in rebellion against his tyranny.
Would you be willing to fight and go to war in defense of your faith, and if so would you be committing a sin in doing so?