Was the Apostolic Tradition of rejecting soldiers from the church ‘permanent’, or was this ‘tradition' briefly enforced?

Here's an answer from a Christian pacifist perspective.

Christian pacifists would point out that the Bible verses you quote focus on what is not said rather than on what is said, i.e. an argument of silence. We highlight what Jesus and the apostles did say about violence: turn the other cheek (Mt 5:39), love your enemies (Mt 5:44), we do not wage war like the world does (2 Cor 10:3) etc. This is strengthen by the fact that pacifism was very widespread in the early church.

Ron Sider writes about this in his new book The Early Church on Killing. In an interview with Christianity Today, he criticizes John Helgeland, quoted in another answer on this page, for cherry-picking quotes istead of looking at the whole picture:

There are works with extensive quotations, but as far as I know nobody has ever tried to collect everything we have extant in one volume. It is overdue given that even the best, most careful just-war historians, like John Helgeland, make sweeping statements that are simply inaccurate when you take the whole body of data together. I'm glad I had the privilege of finally doing it.

More specifically, he criticizes how Helgeland and others think that the reason so many church fathers were against Christian military involvement was because of the risk of idolatry and not that they didn't want to kill:

Their most frequent statement is that killing is wrong. Killing a human being is simply something that Christians don't do, and they'll cite the Micah passage or Jesus' "love your enemies" to support that. But the clear statement that Christians don't kill is the foundation.

The most frequently stated reason that Christians didn't join the army and go to war is that they didn't kill. But it's also true that in Tertullian, for example, idolatry in the Roman army is a second reason for not joining the military. But it's not true that idolatry is the primary or exclusive reason that the early Christians refused to join the military. More often they just say killing is wrong.

On the topic of early Christian presence in the Roman army, Sider says:

First, the evidence we have is modest, and so we have to be careful when drawing conclusions about how many Christians were in the military until the last decade of the third century. It's clear from the Thundering Legion story, which probably goes back to an actual event, that there were at least a few Christians in the military in 173. There is other scattered evidence in the first part of the third century.

It's significant that Origen in the middle of the third century, 248–250, responds to the pagan critic Celsus. Celsus said, If everybody was like you Christians, the Roman Empire would collapse. Origen responded, In fact, if everybody was like us, the Roman Empire would be safe, and we wouldn't need to kill people. So in the middle of the third century, the most prominent Christian author writing at the time responded in a way that only makes sense if Christians by and large didn't join the military.

By the last decade of the third century and the first decade of the fourth, it's clear that there were growing numbers of Christians in the military. Here's how I understand that disconnect between what every extant Christian writer we have says, Christians don't kill, and the growing frequency of Christians in the military: There has always been a disconnect between what Christian teachers have said and what average Christians did.

In addition, historians for the Roman army make it quite clear that you could be in the Roman army for long periods of time in the second, third, and fourth centuries and never be in a battle. There was widespread peace for a lot of this period. One author says you could be in the Roman army for many, many years and never get in a fight beyond the tavern.

Finally, let us hear some of these church fathers ourselves:

Justin Martyr wrote in 160 AD: “We ourselves were well conversant with war, murder, and everything evil, but all of us throughout the whole wide earth have traded in our weapons of war. We have exchanged our swords for ploughshares, our spears for farm tools. Now we cultivate the fear of God, justice, kindness to men, faith, and the expectation of the future given to us by the Father himself through the Crucified One.” (Dialogue with Trypho 110.3.4)

Tatian, (dead c. 185), Justin's disciple, wrote: “I do not wish to be king, I don’t want to be rich, I reject military service. I hate adultery”(The Ante-Nicene Fathers, Alexander Roberts and James Donaldson, Grand Rapids, Michigan, Vol. II, reprint 1979, p. 69)

Athenagoras (133-190) wrote: "What, then, are these teachings in which we are reared? ‘I say to you, love your enemies, bless those who curse you, pray for those who persecute you, that you may be sons of your Father in heaven, who makes his sun to shine on the evil and on the good, and sends his rain on the just and on the unjust . . . Who [of the pagan philosophers] have so purified their own hearts as to love their enemies instead of hating them; instead of upbraiding those who first insult them (which is certainly more usual), to bless them; and to pray for those who plot against them? . . . With us, on the contrary, you will find unlettered people, tradesmen and old women, who, though unable to express in words the advantages of our teaching, demonstrate by acts the value of their principles. For they do not rehearse speeches, but evidence good deeds. When struck, they do not strike back; when robbed, they do not sue; to those who ask, they give, and they love their neighbours as themselves . . . We . . . cannot endure to see a man being put to death even justly." (Legatio 11, 34-35 (Athens, 175))

Irenaeus of Lyon (c. 130-202) wrote: “But the law of liberty, that is, the word of God, preached by the apostles (who went forth from Jerusalem) throughout all the earth, caused such a change in the state of things, that these [nations] did form the swords and war-lances into ploughshares, and changed them into pruning-hooks for reaping the corn, [that is], into instruments used for peaceful purposes, and that they are now unaccustomed to fighting, but when smitten, offer also the other cheek.” (The Ante-Nicene Fathers, Vol. I, reprinted 1977, p. 512)

Tertullian (160-220) wrote: “To begin with the real ground of the military crown, I think we must first inquire whether warfare is proper at all for Christians. … Shall it be held lawful to make an occupation of the sword, when the Lord proclaims that he who uses the sword shall perish by the sword? And shall the son of peace take part in the battle when it does not become him even to sue at law? … Of course, if faith comes later, and finds any preoccupied with military service, their case is different, as in the instance of those whom John used to receive for baptism, and of those most faithful centurions, I mean the centurion whom Christ approves, and the centurion whom Peter instructs; yet, at the same time, when a man has become a believer, and faith has been sealed, there must be either an immediate abandonment of it, which has been the course with many; or all sorts of quibbling will have to be resorted to in order to avoid offending God, and that is not allowed even outside of military service; or, last of all, for God the fate must be endured which a citizen-faith has been no less ready to accept. Neither does military service hold out escape from punishment of sins, or exemption from martyrdom.” (On the chaplet 11)

Origen wrote: “You cannot demand military service of Christians any more than you can of priests. We do not go forth as soldiers.” (Against Celsus VIII.7.3)

And Cyprian (200-258) wrote: “The world is soaked with mutual blood. When individuals commit homicide, it is a crime; it is called a virtue when it is done in the name of the state. Impunity is acquired for crimes not by reason of innocence but by the magnitude of the cruelty.” (To Donatus, chapter 6)

Personally, I think it's quite clear that pacifism was very common in the early church, and that shines light on the teachings of Jesus.