Ephesians 6:12

Ephesians 6:10-20 (see also 1st John 3:8)
Finally, be strong in the Lord and in the strength of His might. Put on the full armor of God, so that you will be able to stand firm against the schemes of the devilFor our struggle is not against flesh and blood, but against the rulers, against the powers, against the world forces of this darkness, against the spiritual forces of wickedness in the heavenly places. Therefore, take up the full armor of God, so that you will be able to resist in the evil day, and having done everything, to stand firm. Stand firm therefore, having girded your loins with truth, and having put on the breastplate of righteousness, and having shod your feet with the preparation of the gospel of peace; in addition to all, taking up the shield of faith with which you will be able to extinguish all the flaming arrows of the evil one. And take the helmet of salvation, and the sword of the Spirit, which is the word of God. With all prayer and petition pray at all times in the Spirit, and with this in view, be on the alert with all perseverance and petition for all the saints, and pray on my behalf, that utterance may be given to me in the opening of my mouth, to make known with boldness the mystery of the gospel, for which I am an ambassador in chains; that in proclaiming it I may speak boldly, as I ought to speak.

So, then, Christians have an enemy, which is the devil. However, if God has decreed whatsoever comes to pass, including whatsoever the devil thinks, says and does, then questions are raised about whether the real enemy is the devil, or whether it is an alleged decree that Calvinists credit to God: 

The Calvinistic, Westminster Confession of Faith: “God from all eternity, did, by the most wise and holy counsel of His own will, freely, and unchangeably ordain whatsoever comes to pass; yet so, as thereby neither is God the author of sin, nor is violence offered to the will of the creatures; nor is the liberty or contingency of second causes taken away, but rather established. Although God knows whatsoever may or can come to pass upon all supposed conditions; yet has He not decreed anything because He foresaw it as future, or as that which would come to pass upon such conditions. By the decree of God, for the manifestation of His glory, some men and angels are predestinated unto everlasting life; and others foreordained to everlasting death.” (Westminster Confession of Faith, III. Of God’s Eternal Decree, emphasis mine) 

John Calvin: “Indeed, the ungodly pride themselves on being competent to effect their wishes. But the facts show in the end that by them, unconsciously and unwillingly, what was divinely ordained is implemented.” (Concerning the Eternal Predestination of God, p.173, emphasis mine) 

John Calvin: “Does God work in the hearts of men, directing their plans and moving their wills this way and that, so that they do nothing but what He has ordained?” (Concerning the Eternal Predestination of God, p.174, emphasis mine) 

John Calvin: “But it is quite frivolous refuge to say that God otiosely permits them, when Scripture shows Him not only willing but the author of them.” (Concerning the Eternal Predestination of God, p.176, emphasis mine) 

​Question: If God is opposed to the devil, but decrees everything that the powers of darkness thinks, says and does, all while encouraging Christians to “pray at all times in the Spirit,” then has such an alleged decree caused God’s kingdom to be divided against itself?

Answer: When refuting the Pharisee’s claim that the origin of Christ’s powers had originated from the devil, Jesus’ proof was: “If Satan casts out Satan, he is divided against himself; how then will his kingdom stand?” (Matthew 12:26) But if Christ has come to destroy the works of the devil that God decreed, then doesn’t the same logic apply? How will the kingdom of God stand, if Christ is arrayed against God’s own decree, unless Christ’s war against the devil is merely an orchestration, and if so, then how is that not an unraveling of Jesus’ own proof-text, in that the devil’s kingdom could hypothetically stand if the devil really wasn’t warring against himself through Christ’s miracles, but was only engaged in a similar orchestration?