This is how I understand it (with Scripture)
Become A Jew?
A Classic error prevalent in Christianity today is that some Christians believe that they are “grafted in” to become “a Jew” when they believe.
This is a great error because Scripture itself speaks of “Jewish believers” & “Gentile believers” (making a distinction in some aspect).
An example is shown below:
No distinction is made between the Jewish believer or Gentile believer with respect to aspects like “Salvation”, “gifts of the Holy Spirit”, “life in the age to come”, “God’s fair dealing toward each/”no” special rights” etc.
A difference is maintained between the Jewish believer & Gentile believer in aspects of “keeping the Law” (see Acts 15 & 21 as my previous post “The Law” explains this Scriptural reasoning as well).
For example whilst the Jewish believers may be required to keep all customs of Moses
“And when they heard it they began glorifying God; and they said to him, “You see, brother, how many thousands there are among the Jews of those who have believed, and they are all zealous for the Law;” (Acts 21:20)
“take them and purify yourself along with them, and pay their expenses so that they may shave their heads; and all will know that there is nothing to the things which they have been told about you, but that you yourself also walk orderly, keeping the Law. ” (Acts 21:24)
Read “carefully” and you will realise that the verses above “only” refer to & speak about “Jewish believers”.
While, a Gentile believer is only required to keep only the following (note the word “but” too):
“But concerning the Gentiles who have believed, we wrote, having decided that they should abstain from meat sacrificed to idols and from blood and from what is strangled and from fornication.” (Acts 21:25)
Full details of what part of” “Law of Moses” to keep for the “Gentile believer”:
The observances for “Gentile believers” as listed as follows by the “approval of the Holy Spirit”:
“For it seemed good to the Holy Spirit and to us to lay upon you no greater burden than these essentials; that you abstain from things sacrificed to idols and from blood and from things strangled and from fornication; if you keep yourselves free from such things, you will do well. Farewell.” (Acts 15:28 – 29)
Many misunderstandings come when we mix or take the Scriptures above and apply it ‘as we like’ to an audience ‘not’ intended.
Remember, the “grafted in” verses do “not” apply here simply because the context of which part is spoken of there (i.e. “the topic”) is “not” which part of the Law of Moses does a Gentile require to keep?
So, the “grafted in” verses Speak of the things (or aspects) to which it is “intended to” (nothing more, nothing less) just like rest of Scripture too.
Shalom & Peace ✌ to you.