Christians agree with Muslims that the goal of all of life (including politics) is theocracy. We agree that if God is Lord of our hearts, he must also be Lord of our state. Furthermore, we have always understood our goal and mission, as stated in the Great Commission, as the gospel of Jesus filling the earth. Our fundamental disagreement with Islam is over which God is Lord. Where Islam claims that Allah is God and Muhammad is his prophet, Christians insist that the Trinity is the one God of the universe, and that he has revealed himself in the person of Jesus Christ. He has been given the name above every name, and it is to him that all things in heaven and on earth must bow (cf. Phil. 2:9-11).
This gives some Christians the “willies” when they remember some of the atrocities of the crusades and other “Christian” states carried out in the name of Christ. But the opposite and far worse error is the assumption that there is some realm that Jesus is not King of. Christians have for too long lived and thought like functional atheists when it comes to politics. The failure of the medieval Church, the crusades, et al was not that they believed Jesus was Lord of the state. The problem was that they came to believe that the weapon of the state (the sword) was to be preferred to the weapon of the Church (the Word of God). The Christian faith declares that God overthrew the powers of sin and darkness in the death and resurrection of Jesus Christ. More powerful than any two-edged sword is the Word of God, the same Word that created the worlds from nothing, the same Word that called Jesus back from the dead. The medieval Church, instead of understanding that self-sacrificing love is the way to glory and dominion, stooped to the rocks and sticks of military coercion. The irony is that in fighting Islam, we stooped to the tactics of Islam.