"The devil is God's devil, always under God's power and control."

Regarding the strife and war which Satan is said to wage with God, it must be understood that Satan cannot possibly do anything against the will and consent of God. . . . Satan is under the power of God, and is so ruled by his authority, that he must yield obedience to it. . . .

As God holds him bound and fettered by the curb of his power, he executes those things only for which permission has been given him, and thus, however unwilling, obeys his Creator, being forced, whenever he is required, to do Him service. . . .

God thus turning the unclean spirits hither and thither at his pleasure, employs them in exercising believers by warring against them, assailing them with wiles, urging them with solicitations, pressing close upon them, disturbing, alarming, and occasionally wounding, but never conquering or oppressing them.

~ John Calvin (Calvin's Institutes 1.14.17–18.)