Jesus answered, "Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God. That which is born of the flesh is flesh, and that which is born of the Spirit is spirit."
#gospelofjohn 3:5-6

The natural born Israelite must be born again, just like the grafted in. In a sense, all men must be grafted into the Root of Jesse, no matter who their parents might be or what religion they were brought up in.