One who is not of the natural line of Jacob is a Gentile...that is my running definition.
As far as christians go, you are either a believing Jew or believing Gentile.
It's a joke when Gentiles parade themselves as Jews, and even sadder when persecution erupts against Jews all of a sudden those Gentiles are 'Christian'.
A Jew is of the tribe of Judah. There were twelve other tribes. Quite often the distinction is made...
2Ki 16:1 In the seventeenth year of Pekah the son of Remaliah Ahaz the son of Jotham king of Judah began to reign.
2Ki 16:2 Twenty years old was Ahaz when he began to reign, and reigned sixteen years in Jerusalem, and did not that which was right in the sight of the LORD his God, like David his father.
2Ki 16:3 But he walked in the way of the kings of Israel, yea, and made his son to pass through the fire, according to the abominations of the heathen, whom the LORD cast out from before the children of Israel.
2Ki 16:4 And he sacrificed and burnt incense in the high places, and on the hills, and under every green tree.
2Ki 16:5 Then Rezin king of Syria and Pekah son of Remaliah king of Israel came up to Jerusalem to war: and they besieged Ahaz, but could not overcome him.
2Ki 16:6 At that time Rezin king of Syria recovered Elath to Syria, and drave the Jews from Elath: and the Syrians came to Elath, and dwelt there unto this day.
The circumstance here is that Israel is at war with the Jews. Here is the definitive scripture on this...
Eze 37:15 The word of the LORD came again unto me, saying,
Eze 37:16 Moreover, thou son of man, take thee one stick, and write upon it, For Judah, and for the children of Israel his companions: then take another stick, and write upon it, For Joseph, the stick of Ephraim, and for all the house of Israel his companions:
Eze 37:17 And join them one to another into one stick; and they shall become one in thine hand.
Eze 37:18 And when the children of thy people shall speak unto thee, saying, Wilt thou not shew us what thou meanest by these?
Eze 37:19 Say unto them, Thus saith the Lord GOD; Behold, I will take the stick of Joseph, which is in the hand of Ephraim, and the tribes of Israel his fellows, and will put them with him, even with the stick of Judah, and make them one stick, and they shall be one in mine hand.
The timeframe is the return of Christ and the beginning of the Millennium.