There are several places in the Bible where God told the Jews not to marry outside of the Israelites
Deuteronomy 7:3–4 (KJV)
"Neither shalt thou make marriages with them; thy daughter thou shalt not give unto his son, nor his daughter shalt thou take unto thy son. For they will turn away thy son from following me, that they may serve other gods: so will the anger of the Lord be kindled against you, and destroy thee suddenly."
And also passages of consequences when it did happen. Ezra 10:18–44 Then there's King Solomon married whoever he wanted and his wives brought idols into the household.
1 Kings 11:1–2, 4
"But king Solomon loved many strange women, together with the daughter of Pharaoh, women of the Moabites, Ammonites, Edomites, Zidonians, and Hittites; Of the nations concerning which the Lord said unto the children of Israel, Ye shall not go in to them, neither shall they come in unto you: for surely they will turn away your heart after their gods: Solomon clave unto these in love. For it came to pass, when Solomon was old, that his wives turned away his heart after other gods: and his heart was not perfect with the Lord his God, as was the heart of David his father."
It wasn't just the Samaritans even the children of Lot and his descendants and by Ishmael and Esau’s descendants—none of them are considered Israelites.
Genesis 19:36–38
"Thus were both the daughters of Lot with child by their father. And the first born bare a son, and called his name Moab: the same is the father of the Moabites unto this day. And the younger, she also bare a son, and called his name Benammi: the same is the father of the children of Ammon unto this day."
Genesis 25:12–18
"Now these are the generations of Ishmael, Abraham’s son, whom Hagar the Egyptian, Sarah’s handmaid, bare unto Abraham... And they dwelt from Havilah unto Shur, that is before Egypt, as thou goest toward Assyria: and he died in the presence of all his brethren."
Genesis 36:1
"Now these are the generations of Esau, who is Edom."
Deuteronomy 23:3
"An Ammonite or Moabite shall not enter into the congregation of the Lord; even to their tenth generation shall they not enter into the congregation of the Lord for ever."
Though Ruth married in, perhaps like today where they have to convert to be considered Jewish, and others like rehab
Ruth 1:16
"And Ruth said, Intreat me not to leave thee, or to return from following after thee: for whither thou goest, I will go; and where thou lodgest, I will lodge: thy people shall be my people, and thy God my God."
“There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus. And if ye be Christ’s, then are ye Abraham’s seed, and heirs according to the promise.” Galatians 3:28–29
“And if some of the branches be broken off, and thou, being a wild olive tree, wert graffed in among them, and with them partakest of the root and fatness of the olive tree; Boast not against the branches. But if thou boast, thou bearest not the root, but the root thee.”
Romans 11:17–18 By faith we are grafted in