Help with Romans 11:20-21

Depends a lot on just how new is the NC, as tend to see it as all new, and that those under it would be the redeemed of the Lord jesus!
That is why would hold to believers baptism....

 

Maybe going further in the context of Romans 11 helps -
25 For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in.

26 And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob:

27 For this is my covenant unto them, when I shall take away their sins.

28 As concerning the gospel, they are enemies for your sakes: but as touching the election, they are beloved for the father's sakes.

29 For the gifts and calling of God are without repentance.

30 For as ye in times past have not believed God, yet have now obtained mercy through their unbelief:

31 Even so have these also now not believed, that through your mercy they also may obtain mercy.

32 For God hath concluded them all in unbelief, that he might have mercy upon all.

33 O the depth of the riches both of the wisdom and knowledge of God! how unsearchable are his judgments, and his ways past finding out!

34 For who hath known the mind of the Lord? or who hath been his counsellor?

35 Or who hath first given to him, and it shall be recompensed unto him again?

36 For of him, and through him, and to him, are all things: to whom be glory for ever. Amen.

 

He's been banned due to heresy...