Let's look at Romans 1:3 in context (NRSV) :
1:1 Paul, a servant of Jesus Christ, called to be an apostle, set apart for the gospel of God, 2 which he promised [or announced] beforehand through his prophets in the holy scriptures, 3 the gospel concerning his Son, who was descended from David according to the flesh 4 and was declared to be Son of God with power according to the spirit of holiness by resurrection from the dead...
Notice the introduction :
Paul, a servant of Jesus Christ, called to be an apostle, set apart for the gospel of God, which he promised [or announced] beforehand through his prophets in the holy scriptures,Paul is describing a gospel previously promised or announced (Greek proepangello προεπηγγείλατο) in the scriptures. This gospel has been announced in scripture, the writing of the prophets. It was there all the time, encoded in scripture, but now Paul has realized it's meaning ("according to the revelation of the mystery kept in silence for long ages but now made known..." Rom. 16:25)
And what does this gospel say?
It describes two features of Christ :
- who was descended from David in the sphere of the flesh
- and was declared to be Son of God with power in the sphere of the spirit ...
Paul is describing the two phases or locations of Christ's experience - one is in the sphere of the spirit (upper heaven), the other is in the sphere of the flesh (lower heavens) where he was crucified by the prince of power of the Air.
In that lower heaven, the 1st heaven, the Air beneath the Moon, where he was crucified, he took on flesh, specifically David's descendent's flesh because that's what scripture fortold.
I know it seems odd that flesh should occupy a heaven, but I think it comes down to it being the LOWER heaven, which is of flesh in Paul's thinking. It doesn't make much sense to modern thinking, but that's how I think Paul saw it.