There is a verse in Romans that has been often glossed over as referring to the unbelievers when it more appropriately refers to believers.

Rom 8:8-11 Those who are in the flesh cannot please God. 9 You, however, are not in the flesh but in the Spirit, if in fact the Spirit of God dwells in you. Anyone who does not have the Spirit of Christ does not belong to him. 10 But if Christ is in you, although the body is dead because of sin, the Spirit is life because of righteousness. 11 If the Spirit of him who raised Jesus from the dead dwells in you, he who raised Christ Jesus from the dead will also give life to your mortal bodies through his Spirit who dwells in you. ESV

There is a great difference between someone “being in” the flesh and the flesh “being in them.” In other words, as a believer I am not in the flesh although the flesh is in me. That is what Paul refers to in his letter to the Galatians:

Gal 5:17 For the desires of the flesh are against the Spirit, and the desires of the Spirit are against the flesh, for these are opposed to each other, to keep you from doing the things you want to do. ESV

The unsaved are “in the flesh” and as such they can’t please God. On the other hand, believers are not in the flesh, they are “in the Spirit;” even though the flesh is in them. So is it possible for one who is in the Spirit to walk “in the flesh?” Can they set their mind on things of the flesh? The answer is absolutely yes and that’s why the first step to understanding practical sanctification is to recognize that fact.

Our justification is sure, certain, and eternal and there is  no question. That is settled by the Word of God. It is the same for our “positional sanctification.” But our “practical sanctification” is what the Holy Spirit is focused on here.

 1 Thess 4:3-5 For this is the will of God, your sanctification: that you abstain from sexual immorality; 4 that each one of you know how to control his own body in holiness and honor, 5 not in the passion of lust like the Gentiles who do not know God; ESV

And God makes us a promise regarding His will :

Phil 1:6 And I am sure of this, that he who began a good work in you will bring it to completion at the day of Jesus Christ. ESV

In the original Greek it makes the point more clearly (it’s a process):

Phil 1:6 He that hath begun a good work in you will keep on perfecting it until the day of Jesus Christ. 

We are not “in the flesh” but it is a fact the “the flesh is in us.” The whole reason for the Holy Spirit making this point is to make it clear that the struggle between the flesh and the Spirt in a believer is evidence that while the flesh is “in us” we are not “in the flesh.”

Our challenge? Unlike the carnal Christian we are to walk in the fact that we are not in the flesh. We are to walk out our victory over the flesh because God has made our sanctification “certain.”

You, however, are not in the flesh but in the Spirit, if in fact the Spirit of God dwells in you.