Did Christians refrain from joining the military our of hatred for the Romans?

Impossible! When St. Luke wrote his Gospel, he addressed it to a ruler and employed the language of love towards them. The apostle Paul also appealed to Caesar (Acts 26:30). In the second century, the scholar Tertullian says in his defense of the Church, “The world persecutes the church, and the church loves the world and serves it.” This illustrates the spirit of the Church and of Christians towards the state, even amidst times of persecution. Finally, the scholar Origen , the dean of the Catechetical School of Alexandria, believed that in the pax Romana (Roman peace) one could see divine help, for this peace facilitated the spread of the Gospel.⁸⁹⁵

895 Origen of Alexandria, Against Celsus 2.30 (ANF 4:443–444).