That is not true. We are always in the flesh. We will be in the flesh until the day that we die.
Scripture shows you don't have enough information on the topic.
First off, not every use of the word "flesh" defines skin and bones.
Often it describes the "fleshly mindedness" of the old, unconverted man.
For instance..."There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the
flesh, but after the Spirit." (Rom 8:1)
We all have skin and bones, but what is it we "walk after"? The things of this world, or the things of God?
One is of the flesh/world, while the other is of God/heaven.
"That the righteousness of the law might be fulfilled in us, who walk not after the
flesh, but after the Spirit. For they that are after the
flesh do mind the things of the
flesh; but they that are after the Spirit the things of the Spirit." (Rom 8:4-5), This is another example of fleshly mindedness/worldly thinking juxtaposed with Godliness/Godly thinking thinking.
"So then they that are in the
flesh cannot please God. But
ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his." (Rom 8:8-9)
Again, Paul isn't writing of skin and bones, but about the goals in the mind of the Godly oriented converted.
So, back to Rom 7..."For when we
were in the
flesh, the motions of sins, which were by the law, did work in our members to bring forth fruit unto death." (Rom 7:5)
This is a reference to his past, (and ours, if we are now in Christ), walking after the things of this world and not after God.
"For I know that in me (that is, in my
flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not." (Rom 7:18)
Paul's body is not the topic.
His mind, and what it was once oriented after, is the topic.
"I thank God through Jesus Christ our Lord. So then with the mind I myself serve the law of God; but with the
flesh the law of sin." Rom 7:25)
This is the summation of Paul's Rom 7 experiences, first as a Pharisee unsuccessfully trying to satisfy God with Law keeping, and then as a new man free of the "fleshly" oriented world.
In the "flesh" he couldn't successfully serve God.
The new Paul, raised with Christ to walk in newness of life, (Rom 6:4), can successfully serve the Law of God.
I sometimes wish the KJV would have said "skin and bones" instead of "flesh" so there wouldn't be such confusion.