Early Christians faced several problems.
First of all Imperial Rome didn’t have a separate police force (although there was a “watch” in the city itself.) In most of the Empire the laws were enforced by the Army – note that it was Roman Soldiers, not corrections officials or sheriffs, who crucified Christ. During periods of persecution, the military carried out the persecutions.
Second, soldiers were automatically members of a military cult. There were pagan ceremonies and sacrifices that were obligatory. (Read Tertullian “On Wearing the Military Chaplet.”)
At the same time, however, the Army was the Empire’s defense against the chaos of invading barbarians. And Christians routinely prayed for the success of the Emperor’s armies.
Again, soldiers would convert. This raised another problem – what should a convert do? Was he obliged to desert, or disobey orders? If so, how would this reflect on the law-abiding Christian community.
What about times when soldiers were ordered to do things that were clearly forbidden by the Church – like massacreing civilians?
All these problems were not based on a pacifist bent, but on contradictions engendered by being members of a religion that was both law-abiding and illegal.
Once Christianity was legalized by the Edict of Milan (313 AD) the contradictions vanished.