Because there is no "which Israel" Before Paul they were not allowed convert unless they were circumcised and obeyed the Mosaic Law. ISRAEL DID NOT EXIST DURING THE NEW TESTAMENT!!!!
If Israel did not exist during the New Testament time period, then how do you interpret the following passage that refers to an Israel that has existed during the entire New Testament time period, but not all who physically descend from the physical nation of Israel are part of it?
Romans 9:6 It is not as though God’s word had failed.
For not all who are descended from Israel are Israel. 7 Nor because they are his descendants are they all Abraham’s children. On the contrary, “It is through Isaac that your offspring will be reckoned.” 8
In other words, it is not the children by physical descent who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring.
Paul refers to an Israel here that existed at that time and consisted of "God's children" who are "the children of the promise" and are "regarded as Abraham's offspring". You are contradicting Paul by saying there is no Israel that exists during the New Testament time period.
You are showing a very high level of ignorance when it comes to what scripture teaches. How did you come to believe what you believe (pre-trib dispensationalism)? Did someone teach you?