The Church of Christ is continually represented under the figure of an army; yet its Captain is the Prince of Peace; its object is the establishment of peace, and its soldiers are men of peaceful disposition. The spirit of war is at the extremely opposite point to the spirit of the gospel. 

Yet nevertheless, the church on earth has, and until the second advent must be, the church militant, the church armed, the church warring, the church conquering. And how is this?

It is in the very order of things that so it must be. Truth could not be truth in this world if it were not a warring thing, and we should at once suspect that it were not true if error were friends with it. The spotless purity of truth must always be at war with the blackness of heresy and lies. 

-C.H. Spurgeon