The strongest argument for Christian pacifism is the New Testament. The simplist argument is Jesus said "love your enemies" and then described that loving includes not harming, not killing. To love someone is to do good to them; to kill someone is the opposite of doing good (See Mark 3:4).
The reason why Jesus and the others didn't get preach against soldier's killing is because, for the most part, they didn't. They acted as a police force in Judea, not a military killing machine-- the Romans who did that work were sent elsewhere.
The NT as a whole argues against the whole miltary machine. Revelation-- getting back to topic-- is an excellent example. The whole book glorifies martyrs-- those who submit to death rather than have their faith be compromised-- and assumes that those in the military machine are those who kill the martyrs. It's a pretty fair assumption. But even if one disagrees with that assumption, we have to admit that those who participate in the society of the Beast-- the military empire-- and those who participate in Babylon-- the commercial support of empire-- are condemned. If we accept this as Scripture, we have to take it seriously and not just dismiss its radical notions.
BTW, the basis for saying that the early church was anti-military isn't based on Roman records but on Christian documents. We have martyrologies of soldiers becoming Christians and then were killed because they refused to participate in warfare. We have early Christian authors saying that participating in the military in unacceptable for a Christian. To see these sources, you can find them in Eberhard Arnold's excellent book, "The Early Christians."