Totten, it says "by the Cross" not "at" the Cross.
In other words, the Cross has made possible Paul's preaching among the Gentiles.
And his preaching among the Gentiles did not begin until a point after Israel was concluded having continued in the uncircumcision of their fathers - against all three members of the Godhead now - Acts 4-7; Acts 9; Gal. 1, 1 Thess. 2.
Study out the Gentile assemblies that resulted in the area of Tarsus of Cilicia after Paul headed there in Acts 9.
This is why Barnabas went and brought Paul to Antioch in Acts 11 - because he was already the Apostle of the Gentiles with Gentile assemblies under him before Antioch - and all this was after Acts 7.
You see these assemblies in Acts 15.
None of that was "at" the Cross. Rather "by" it.
Ephesians 2 is Romans 1-3 just as Romans 15 is Ephesians 3.
All of which are AFTER Romans 3:21's " BUT NOW..."
This was made possible "by" the Cross, but did not begin until Romans 3:21's "But now..."
Have it your way. By the cross, not any time after. And the matter was revealed to the apostles and prophets when they gathered to question Peter for fellowshipping with Gentiles.
Paul was brought into by Barnabas to a Jew/Gentile church at Antioch.
I am the first to admit that the Jerusalem crowd were totally inept and incapable of carrying the church forward to the Gentiles.
Paul's call and ministry [dispensation] was vital. But that does not alter the fact we Gentiles are brought into the commonwealth of Israel, made fellow citizens with them, co-heirs of their promises, grafted into their olive tree.
If you lop off the Jews then with who are we fellow citizens with?...it is THEIR commonwealth, who are we co-heirs and partakers with? the promises belong to them, The olive into which we are grafted is theirs.
And this is just what we find for you say these books [containing the promises] are not for us....the Amillennialists say exactly the same, they also lop off the Jews.
The only things that are lopped off are the law of commandments and ordinances...THAT is why Paul is so crucial to us, that is why he is OUR apostle.
It was the separation that was made when Peter withdraw from the Gentiles that got Paul so incensed....but now YOU are separating from Peter, there must be no separation if the gospel is to be preserved IN IT'S FULNESS for us.
....only we are made FULL partakers without having to obey commandments