From the Douay-Rheims Bible: Romans, Chapter 11
[19] Thou wilt say then: The branches were broken off, that I might be grafted in. [20] Well: because of unbelief they were broken off. But thou standest by faith: be not highminded, but fear.
[20] Thou standest by faith. Be not highminded, but fear: We see here that he who standeth by faith may fall from it; and therefore must live in fear, and not in the vain presumption and security of modern sectaries.
[21] For if God hath not spared the natural branches, fear lest perhaps he also spare not thee. [22] See then the goodness and the severity of God: towards them indeed that are fallen, the severity; but towards thee, the goodness of God, if thou abide in goodness, otherwise thou also shalt be cut off. [23] And they also, if they abide not still in unbelief, shall be grafted in: for God is able to graft them in again. [24] For if thou wert cut out of the wild olive tree, which is natural to thee; and, contrary to nature, were grafted into the good olive tree; how much more shall they that are the natural branches, be grafted into their own olive tree? [25] For I would not have you ignorant, brethren, of this mystery, (lest you should be wise in your own conceits), that blindness in part has happened in Israel, until the fulness of the Gentiles should come in.
[22] Otherwise thou also shalt be cut off: The Gentiles are here admonished not to be proud, nor to glory against the Jews: but to take occasion rather from their fall to fear and to be humble, lest they be cast off. Not that the whole church of Christ can ever fall from him; having been secured by so many divine promises in holy writ; but that each one in particular may fall; and therefore all in general are to be admonished to beware of that, which may happen to any one in particular.
Regardless, I’m still not seeing Bible support for Christians worshipping with Jews (which is what the OP is about).
[26] And so all Israel should be saved, as it is written: There shall come out of Sion, he that shall deliver, and shall turn away ungodliness from Jacob. [27] And this is to them my covenant: when I shall take away their sins. [28] As concerning the gospel, indeed, they are enemies for your sake: but as touching the election, they are most dear for the sake of the fathers.