I believe most of the violence they faced in the first 300 years derived from persecution which is a different set of rules than life in general. The bible says that God is the same, he changes not, so have you read the OT? Factually, Israel was the invading army not a defender but had to continue the fight to maintain what God had blessed them with. So, if Israel could fight to protect themselves than it is illogical to think that the church should not do likewise.
Except Jesus came along and said, love your enemies, do good to those who persecute you, turn the other cheek, etc. The early church didn't make a distinction between religious persecution and any other offence.
Clement of Alexandria, Fragments.
Above all, Christians are not allowed to correct with violence the delinquencies of sins. For it is not those that abstain from wickedness from compulsion, but those that abstain from choice, that God crowns.
Early Church Fathers - – Ante-Nicene Fathers: The Writings of the Fathers Down To A.D. 325.
Lactantius, Book 5, Chapter 23.
But the just and wise man, because he deems all these things as human, as it has been said by Lælius, and his own goods as divine, neither desires anything which belongs to another, lest he should injure any one at all in violation of the law of humanity; nor does he long for any power or honour, that he may not do an injury to any one. For he knows that all are produced by the same God, and in the same condition, and are joined together by the right of brotherhood. But being contented with his own, and
that a little, because he is mindful of his frailty, he does not seek for anything beyond that which may support his life; and even from that which he has he bestows a share on the destitute, because he is pious; but piety is a very great virtue. To this is added, that he despises frail and vicious pleasures, for the sake of which riches are desired; since he is temperate, and master of his passions. He also, having no pride or insolence, does not raise himself too highly, nor lift up his head with arrogance; but he is calm and peaceful, lowly and courteous, because he knows his own condition. Since, t
herefore, he does injury to none, nor desires the property of others, and does not even defend his own if it is taken from him by violence, since he knows how even to bear with moderation an injury inflicted upon him, because he is endued with virtue; it is necessary that the just man should be subject to the unjust, and that the wise should be insulted by the foolish, that the one may sin because he is unjust, and the other may have virtue in himself because he is just.
Early Church Fathers - – Ante-Nicene Fathers: The Writings of the Fathers Down To A.D. 325.
Tertullian, First Apology, Chapter 37
We are but of yesterday, and we have filled every place among you—cities, islands, fortresses, towns, market-places, the very camp, tribes, companies, palace, senate, forum,—we have left nothing to you but the temples of your gods. For what wars should we not be fit, not eager, even with unequal forces,
we who so willingly yield ourselves to the sword, if in our religion it were not counted better to be slain than to slay?
Early Church Fathers - – Ante-Nicene Fathers: The Writings of the Fathers Down To A.D. 325.