How do you know a Christian? Is there a way to tell who is and who isn’t? Do you quiz them on their beliefs to see how well they line up with the Nicene Creed? Do you ask them if they’ve invited Jesus into their hearts?

One answer is to check their behavior. If becoming united to Christ changes us, then one should expect to see those changes lived out in everyday life. A number of the Church Fathers suggested a test like this one, though the specific change they were looking for may come as a surprise.

In the second century, in response to charges of cannibalism from non-Christians (the Eucharist is said to be the body and blood of Christ), Athenagoras pointed out that in order to eat human flesh, Christians would first have to be willing to kill. However, even their worst enemies knew that Christians rejected violence. Christians “cannot endure even to see a man put to death” even if the condemned person deserved such a fate. Christians refused to witness the gladiatorial spectacles, argued that abortion was tantamount to murder, and stood out from the morality of the day by making the audacious claim that “those who expose [infants] are chargeable with child-murder” (A Plea for the Christians, 35).

The weight of evidence from the early church shows that this Christian attitude toward killing also applied to the field of battle. More than one early church writer has claimed that Christian rejection of violence was a fulfillment of Old Testament prophecy and definitive proof of Jesus’ messiahship.

In his First Apology, the second-century Christian apologist Justin quotes Isaiah’s famous prophecy about the nations beating their swords into gardening tools because they have received the word of God that they should not “learn war any more” (Isa. 2:4).

Justin argued that this passage had already been fulfilled, at least partially, in his own day: “From Jerusalem there went out into the world men, twelve in number, and these illiterate, of no ability in speaking: but by the power of God they proclaimed to every race of people that they were sent by Christ to teach to all the word of God; and we who formerly used to murder one another do not only now refrain from making war upon our enemies, but also, that we may not lie or deceive our examiners, willingly die confessing Christ” (First Apology, 39).

Here is Justin’s argument in short: Jesus taught nonviolence and the apostles carried that message forward to the gentiles; therefore those who accepted Christ and his teaching naturally embraced his way of peace. Since this teaching went to different nations and ethnicities, Isaiah’s prophecy had already begun to be fulfilled.

In his Dialogue with Trypho, Justin elaborates that Jesus’ command to love our enemies had been “predicted by Isaiah” (ch. 85), and found its fulfillment in Christians, who, though formerly “were filled with war, and mutual slaughter, and every wickedness, have each through the whole earth changed our warlike weapons – our swords into plowshares.” With transformed tools in hand, “we cultivate piety, righteousness, philanthropy, faith, and hope, which we have from the Father himself through him who was crucified” (ch. 110).

Irenaeus repeats this description of Christians, but puts more emphasis on Jesus as the agent of fulfillment. He saw God’s word as having “caused such a change in the state of things” that those who had beaten their swords into plowshares “are now unaccustomed to fighting, but when smitten, offer also the other cheek.” This change was undeniable proof that “the prophets have not spoken these things of any other person, but of him who effected them. This person is our Lord” (Against Heresies, book 4, ch. 34). Irenaeus not only treats Christian nonviolence as a fulfillment of prophecy, but implies that if Christians do not follow this practice, people will doubt that Jesus is the prophesied Messiah we claim he is.

Though in some of his earlier writing Tertullian seems to be aware of Christians serving in the army without offering a strong critique, even at this stage he finds in Isaiah’s plowshares prophecy a prediction which points to “the new law” of Christ: “The new law’s practice was to point to clemency, and to convert to tranquility the pristine ferocity of swords and lances, and to remodel the pristine execution of war upon the rivals and foes of the law into the pacific actions of plowing and tilling the land” (Against the Jews, 3).

These are not the only examples of the Church Fathers citing Isaiah as having found fulfillment in the nonviolent practices of Christians. Ron Sider’s The Early Church on Killing: A Comprehensive Sourcebook on War, Abortion, and Capital Punishment collects every extant reference from the early church to the morality of taking human life. One of the key elements of Sider’s summary of all these primary sources, along with a clear conclusion that “killing is wrong” and “Jesus teaches us to love enemies,” is that “Christ fulfills Isaiah’s messianic prophecies of peace.” Sider writes: “With one exception, every Christian writer who cites these prophecies also explicitly concludes that Christ fulfilled them and that therefore Christians do not engage in war.”

The early church witness is generally seen as indispensable for determining the contours of Christian belief and practice. Some will argue that the church fathers are not inspired in the same way that scripture is: they could be mistaken in their understanding of Isaiah and the manner of its fulfillment. Nevertheless, their unity of opinion on this point, in conjunction with Jesus’ teachings on nonviolence, does have weight, and we should accept their rebuke with humility. Anyone who claims Jesus as their Lord and as the fulfillment of God’s promises in the Hebrew scriptures ought to be a witness to the reality of his kingdom.