There are a number of reasons why Christians cannot join the military, a few of which are given below.
1. Although Christians are to be subject the the authority of the governments of the countries they live in (Rom. 13:1; 1 Peter 2:13-14), Christians are ‘strangers and pilgrims’ in this world (Heb. 11:13-14) and have their actual citizenship as a member of God’s kingdom (Phil. 3:20), under the reign of his son, the Lord Jesus. To fight in another country’s war would be to leave the allegiance of God’s kingdom (Heb. 11:15-16; John 18:36).
2. At the moment, Jesus’ command is that his subjects do not engage in warfare (John 18:36; Mat. 26:52). When Jesus returns to the earth, it’s possible that — under his rule — the saints will be involved in warfare at some time (Ps. 149), but, until then, the command of their king is not to fight.
3. Disciples of Jesus are to follow his teaching. In relation to this topic Jesus said, for example, that Christians should do to others as they would have done to themselves (Mat. 7:12), and, perhaps even more powerfully, that they should love their neighbours as themselves (e.g. Mat. 22:39) — these teachings alone rule out use of force like that of the military.
4. Jesus’ disciples are to follow his example. For example, when Jesus was attacked he did not resist with force (e.g. Mat. 26:47-53), so neither should we:
…to this you have been called, because Christ also suffered for you, leaving you an example, so that you might follow in his steps. He committed no sin, neither was deceit found in his mouth. When he was reviled, he did not revile in return; when he suffered, he did not threaten, but continued entrusting himself to him who judges justly. [1 Peter 2:21-23]
5. Christians are to live at peace with others (Rom. 12:18; Heb. 12:14).
What about non-combatant roles?
Point 1. above still applies, regardless of the role taken in the military. The other points also still apply because involvement in non-combatant roles in the military is still involvement in an organisation that functions on ungodly principles and does unChrist-like acts.
Q. What can be done about evil, then?
A.1 Use different methods.
Sometimes the goal of a military organisation is admirable: it’s good to want evil to be removed from the world. However, the methods of military organisations (e.g. the use of force) are not commendable. What can be done about evil in the world, then? The answer, seen in Jesus, is to use different methods. Jesus did not use force, but he overcame evil, nontheless. His methods for overcoming evil were, e.g., bringing healing, providing food and educating people (this last one — with regard to the education Jesus was giving people: i.e., teaching them the gospel — is the most important). Jesus overcame evil by doing good. And this is the method we should use, too.
Rejoice in hope, be patient in tribulation, be constant in prayer. {13} Contribute to the needs of the saints and seek to show hospitality. {14} Bless those who persecute you; bless and do not curse them. {15} Rejoice with those who rejoice, weep with those who weep. {16} Live in harmony with one another. Do not be haughty, but associate with the lowly. Never be wise in your own sight. {17} Repay no one evil for evil, but give thought to do what is honorable in the sight of all. {18} If possible, so far as it depends on you, live peaceably with all. {19} Beloved,never avenge yourselves, but leave it to the wrath of God, for it is written, “Vengeance is mine, I will repay, says the Lord.” {20} To the contrary, “if your enemy is hungry, feed him; if he is thirsty, give him something to drink; for by so doing you will heap burning coals on his head.” {21} Do not be overcome by evil, but overcome evil with good. [Rom. 12:12-21]
A.2 Tackle it at the source.
The source of moral evil in the world is, collectively and individually, ourselves (e.g. Mark 7:20-23; James 1:14-16). So often we follow our own selfish desires instead of looking to the needs of others — or even, we follow our selfish desires at the cost of other people! If, dying with Jesus and rising to a new life (Rom. 6:1-13), we make it our aim to put away our selfish desires and, instead, live a life of love for others (Eph. 5:1-2ff.), then we will, with God’s help, have made a step towards reducing the evil in the world. Imagine if everyone did this! One day, when Jesus comes back, this will be a reality (1 Cor. 15:23,49,52; 1 John 3:2), and it will make for a glorious world.
They shall not hurt or destroy in all my holy mountain; for the earth shall be full of the knowledge of the LORD as the waters cover the sea. [Isa. 11:9 (read the whole of Isa. 11 for a beautiful picture of the kingdom)]