There is no such thing. There are Christians who were formally gentiles just as there are Christians who were formally Israelites or who were formally Jewish. However, as pointed out in scripture, there are only Christians.
Gal_3:28 There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus.
I've shown, from scripture, that the new covenant is ONLY for the house of Israel and the house of Judah. We join ourselves to Israel when we enter into the new covenant with Christ as the mediator.
Heb_9:15 And for this reason He is the Mediator of the new covenant, by means of death, for the redemption of the transgressions under the first covenant, that those who are called may receive the promise of the eternal inheritance.
I'm not sure where you're getting your information from but since you're not quoting scripture I'll assume it's non-biblical tradition. But in context of scripture your statement is meaningless.