This was the introduction to a sermon Charles Spurgeon preached back in 1858.

“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 a 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. I say again, it would cast a suspicion upon its own nature; we should feel at once that it was not true, if it were not an enmity with the false. And so at this present time, the church of Christ, being herself the only incarnation of truth left upon this world, must be at war with error of every kind of shape; or if she were not, we should at once conclude that she was not herself the church of the living God. It is but a rule of nature that holiness must be at enmity with sin. That would be but a mock purity which could lie side by side with iniquity and claim its kinship. “Shall the throne of iniquity have fellowship with thee?” Shall Christ and Belial walk together? Shall the holy be linked with the unholy? If it were so, beloved, we might then not only suspect that the church was not the holy, universal and apostolic church; we might not only suspect it, but we might beyond suspicion pronounce a verdict upon her, “Thou art no more Christ’s bride; thou art an antichrist, an apostate. Reprobate silver shall men call thee, because thou hast not learned to distinguish between the precious and the vile.” Thus, you see, if the church be a true church, and a holy church, she must be armed: there are so many untrue things and unholy things, that she must be perpetually with her sword in her hand, carrying on combat against them. And every child of God proveth by experience that this is the land of war. We are not yet come to the time when every man shall sit under his vine and under his fig tree, none daring to make him afraid. The mountains do not bring peace to the people, nor the little hills righteousness. On the contrary, the children of God hear the sound of war; the shrill clarion is constantly sounding in their ears; they are compelled to carry with them the sword and the shield, and constantly to gird their armor on, for they are not yet come to the land of peace; they are in an enemy’s country, and every day will convince them that such is their position.”