In the encampment of the Saracens they asked St. Cyril: "How could Christians wage war and at the same time keep the commandment of Christ about praying to God for their enemies?"
To that, St. Cyril replied: "If two commandments were written in one law and given to men for fulfilling, which man will be a better follower of the law: the one who fulfills one commandment or the one who fulfills both?"
To that, the Saracens replied: "Undoubtedly, he who fulfills both commandments."
St. Cyril continued: "Christ our God commands us to pray to God for those who persecute us and even to do good to them; but, He also said to us: greater love cannot be shown in this world than if one lay down his life for his friends." "Greater love has no man than this, that a man lay down his life for his friends" (St. John 15:13). That is why we bear the insults which our enemies do to us individually and we pray to God for them; and, as a society, we defend one another and give up our lives, that you would not somehow enslave our brethren, would not enslave their souls with their bodies and would not kill them in body and soul.
— The Prologue of Ohrid, May 11
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This is a false argument, and it does NOT justify warfare. Take a closer look at what Jesus said:
This is my commandment, That ye love one another, as I have loved you.
Greater love hath no man than this, that a man lay down his life for his friends.
Ye are my friends, if ye do whatsoever I command you.
John 15:12-14 KJV
Read John Chapter 15 in its entirety. It is NOT a justification for war. It is a call to martyrdom. The greatest love is to lay down your life for your friend, not to kill someone else because you think you are "defending" him.
Consider the example of Jesus Himself. He easily could have defeated the Romans and all of His enemies, but He did not. He gave His life for His friends and for all of us. How can His example POSSIBLY be used to justify killing anyone, whether in warfare or in self-defense?