That Paul understood the Church, the Body of Christ which included Gentiles who believed in Christ as well as jews, is Israel, we look at a sermon he preached in Acts 13:23,32-35
23 From this man’s seed, according to
the promise,
God raised up for Israel a[a] Savior—Jesus—
Now Paul at that time was speaking in a jewish synagogue, but notice that some Gentiles heard this message, that God raised up for Israel a Saviour, Jesus, who is going to save His People from their sins Matt 1:21 thats a promise. Notice again in that same sermon Pauls words again about God raising up Jesus
32 And we declare to you glad tidings—that promise which was made to the fathers. 33
God has fulfilled this for us their children, in that He has raised up Jesus. As it is also written in the second Psalm:
‘You are My Son,
Today I have begotten You.’
34 And that He raised Him from the dead, no more to return to [
i]corruption, He has spoken thus:
‘I will give you the sure [
j]mercies of David.’
35 Therefore He also says in another
Psalm:
‘You will not allow Your Holy One to see corruption.’
Now notice starting at Vs 42 of the same chapter at the conclusion of that sermon, some Gentiles came to Paul, no doubt moved by the Spirit of God and said:
42 So when the Jews went out of the synagogue,
the Gentiles begged that these words might be preached to them the next Sabbath.
The Gentiles wanted to hear the same Gospel Message preached to them as it was preached to the jews, and how Jesus was raised according to promise, a Gospel promise, a Saviour to Israel. See that was the Mystery unfolding, that the Gentiles were joint heirs with jews of the same Gospel promises to Israel Eph 3:1-6
For this reason I, Paul, the prisoner of Christ Jesus for you Gentiles— 2 if indeed you have heard of the [
a]dispensation of the grace of God which was given to me for you, 3 how that by revelation He made known to me the mystery (as I have briefly written already, 4 by which, when you read, you may understand my knowledge in the mystery of Christ), 5 which in other ages was not made known to the sons of men, as it has now been revealed by the Spirit to His holy apostles and prophets:
6 that the Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ through the gospel
The Gospel promise spoken of back in Acts 13:23 on how Jesus was raised a Saviour to Israel ! Gentiles were included in that Israel, with Christ as their head, who was raised from the dead.
This is the Hope of Israel Paul was in chains for Acts 28:20
For this reason therefore I have called for you, to see
you and speak with
you, because for the hope of Israel I am bound with this chain.”