Added......
I don't think it's the case that early Christians were pacifists in the modern day sense and I cited the writing as evidence. August of Hippo would fight, if we are to believe his words.
I've heard that Augustine urged soldiers to repent of killing before the battle, though I am having difficulty finding a source for that. Regardless he was a early proponent of what we call Just War theory, though I don't think he articulated the points as clearly as they came to be known. I'm fine with Just War Theory
as theory, the only problem is that there has
never been a just war based on those standards.
As for other ECFs, it's pretty clear where they stood.
"[W]e who were filled with war, and mutual slaughter, and every wickedness, have each through the whole earth changed our warlike weapons,— our swords into ploughshares, and our spears into implements of tillage, —and we cultivate piety, righteousness, philanthropy, faith, and hope, which we have from the Father Himself through Him who was crucified" - Justin Martyr, Dialogue with Trypho
“Hitherto I have served you as a soldier; allow me now to become a soldier to God. Let the man who is to serve you receive your donative. I am a soldier of Christ; it is not permissible for me to fight.”
- Martin of Tours (Not sure if he technically qualifies as ECF)
[W]e have learned, not only not to return blow for blow, nor to go to law with those who plunder and rob us, but to those who smite us on one side of the face to offer the other side also, and to those who take away our coat to give likewise our cloak" - Athenagoras, A Plea for the Christians
(After quoting Matthew 5:44-45)"But among us you will find uneducated persons, and artisans, and old women, who, if they are unable in words to prove the benefit of our doctrine, yet by their deeds exhibit the benefit arising from their persuasion of its truth: they do not rehearse speeches, but exhibit good works; when struck, they do not strike again; when robbed, they do not go to law; they give to those that ask of them, and love their neighbors as themselves." - Athenagoras, A Plea for the Christians
"[T]he Christian does no harm even to his foe...Hippias is put to death laying plots against the state: no Christian ever attempted such a thing in behalf of his brethren, even when persecution was scattering them abroad with every atrocity. But it will be said that some of us, too, depart from the rules of our discipline. In that case, however, we count them no longer Christians;" - Tertullian, Apology, ch 46
I can find many others.