Interesting Comments from Herman Ridderbos

Ridderbos points that Paul's argument for the universality of the new covenant includes an appeal to Israel's own distinctive confession that God is one God--a confession that the Jews considered to be their own, and something that kept them separate from the gentiles. (By universality, Ridderbos means that the covenant is without distinction--both Jews and gentiles are included.)

On Romans 3:29,30

Or is God the God of Jews only? Is he not the God of Gentiles also? Yes, of Gentiles also, since God is one. He will justify the circumcised....and the uncircumcised....(ESV)

and 1 Timothy 2:4,5,6

who desires all people to be saved..... For there is one God, and there is one mediator between God and men, the man  Christ Jesus,  who gave himself as a ransom for all...(ESV)

Ridderbos says:

From the fact that God is one and that there is no other God than he who has revealed himself to his people Israel, it is therefore concluded that the God known and worshipped by Israel must also be the God of the gentiles. Precisely that which was the ground for their particularism - apart from Israel's God there is no God - here become the ground for the universalism: all men have to do with one God in judgment and in grace. It is that God who judges without respect of persons and will render to every man according to his works (Rom. 2: 6, 11), but who will validate the faith of the circumcised and uncircumcised equally (Rom. 3:30). As the one God he has to do, not with Jews only, but with all men (Rom. 2:9, 16; 3:28) Because God is one the knowledge of his grace is also a knowledge that concerns all men. As apart from him there is no Judge, so apart from him there is no Deliverer and Saviour, and his salvation has to do with all men (1 Tim. 4:10; Tit. 2:11). For this reason there is one Mediator who is the Mediator between God and all men, and the Mediator is so very emphatically called the man Christ Jesus, who gave himself for all (all men without social or national distinction, without distinction of any kind).

--From Paul: An Outline of His Theology, p. 339.

It seems obvious to me now, but I'd never noticed before the exact argument Paul is making: since God is one God, he must be the Saviour of all people.