Peter commands Christians to have a mind that is resolute and to resist Satan by being firm in their faith. Such resistance causes the devil to “flee from you” (James 4:7). Resist means “to take a stand against,” and to be firm is to make that stand solid (the Greek is stereos, from which comes the English stereo, meaning “solid,” or balanced at both ends). That is done by being solidly fixed on the faith (te pistei), which is biblical revelation. It is the whole body of revealed truth contained in Scripture (cf. Gal. 1:23; Eph. 4:5, 13; Phil. 1:27; 1 Tim. 4:1). This is a call to know and believe sound doctrine, to be discerning in distinguishing truth from error, and to be willing to defend the truth and expose error. Jude’s call is most appropriate in this connection: “Beloved, while I was making every effort to write you about our common salvation, I felt the necessity to write to you appealing that you contend earnestly for the faith which was once for all handed down to the saints” (Jude 3). It is that “once-for-all” faith which is the inscripturated revelation of God and constitutes the faith on which believers stand solidly and from which they continually resist Satan. This strong stand is the result of the faithful leading of shepherds in the church, as Paul indicates in Ephesians 4:11–14,

And He gave some as apostles, and some as prophets, and some as evangelists, and some as pastors and teachers, for the equipping of the saints for the work of service, to the building up of the body of Christ; until we all attain to the unity of the faith, and of the knowledge of the Son of God, to a mature man, to the measure of the stature which belongs to the fullness of Christ. As a result, we are no longer to be children, tossed here and there by waves and carried about by every wind of doctrine, by the trickery of men, by craftiness in deceitful scheming.

Since Satan is a liar (John 8:44; cf. Gen. 3:1; 2 Thess. 2:9) and a deceiver (Rev. 20:7–8), the only sure way to stand up against him is by faithful obedience to biblical truth. The battle is a spiritual one, in the supernatural realm, as Paul notes:

For though we walk in the flesh, we do not war according to the flesh, for the weapons of our warfare are not of the flesh, but divinely powerful for the destruction of fortresses. We are destroying speculations and every lofty thing raised up against the knowledge of God, and we are taking every thought captive to the obedience of Christ. (2 Cor. 10:3–5)

“Speculations” are satanic ideologies, ideas, theories, religious philosophies, and systems of thought “raised up against the knowledge of God”; that is, they are anti-biblical viewpoints that have people captive as if they were imprisoned in a great fortress. Christians cannot smash those ideas with human ingenuity, but only with biblical truth—“taking every thought captive to the obedience of Christ.” Only when someone has the mind of Christ on a matter is he rescued from such ideas.