Please refer to Romans 2:28,29 and the several verses preceding these two for more clarity. Jesus was a Jew because his lineage was descent from the tribe of Juda. Jesus being so born and being God in the flesh carried the term Jew to a different height, it's highest in fact, as it was carried from the secular to the spiritual. The scriptures clearly teach that those who accept Christ as Savior become His brother/friends( John 15:15) and also sons of God(1 Jo 3:1,2). If Jesus then was a Jew of the highest order, as the eternal high priest and as sons of God also by adoption( Ro. 8:15,Gal. 4:5, Eph. 1:5) then we too are Jews. As our soul was adopted into the family of God at our rebirth so to wait we for the same effect for our body(Ro. 8:23). Understand also that in eternity where Jesus lived and lives there is not such distinction but as God chose the little nation of Israel out of all nations on earth to call his and Judah being one of the twelve patriarchs that rose to be the twelve tribes that comprised the nation of Israel, so then did God choose the one tribe, that of Judah from which His only begotten Son should spring.
Jesus made it clear He did not come to add to the existing Jewish religious system He came as a mediator of a 'new and living way' through which people can be reconciled to God
Mat 9:16,17 No one puts a piece of unshrunk cloth on an old garment; for the patch pulls away from the garment, and the tear is made worse. Nor do they put new wine into old wineskins, or else the wineskins break, the wine is spilled, and the wineskins are ruined. But they put new wine into new wineskins, and both are preserved."
Heb 10:19,20 Therefore, brethren, having boldness to enter the Holiest by the blood of Jesus, by a new and living way which He consecrated for us, through the veil, that is, His flesh,
A Christian is a new creation and does not have to become a Jew.
Jews are an ethno-religious group, which means that a person can
be either an "ethnic Jew", meaning that this person was born to a
Jewish family, and/or a "religious Jew", meaning that this person
believes the specific religious
tenets of Judaism. Jesus was a Jew because he was born into a
Jewish family, making him, at the very least, an ethnic Jew.
However, most people who sanctify Jesus, be they Christian or Muslim, are neither ethnic Jews or religious Jews. They are not part of that grouping. The core message of Christianity is: "God made Himself incarnate through His Son, the Messiah, who sacrificed Himself on Earth as atonement for all of humanity's sins." None of that has anything to do with Judaism. The core message of Islam is: "God gave prophecy to an Ishmaelite to write a new holy book to supersede the Bible." This is a direct refutation of Judaism, which holds that the Bible is an eternal correct source of religious law.