According to who? There is a word in the nt that is translated "tribes". It's used in this verse:
Mat 19:28 And Jesus said unto them, Verily I say unto you, That ye which have followed me, in the regeneration when the Son of man shall sit in the throne of his glory, ye also shall sit upon twelve thrones, judging the twelve tribes of Israel.
Transliterated it is "phylē" and it means:
A tribe.
a) in the NT all the persons descending from one of the twelve sons of the patriarch, Jacob
2) a nation, people
When a tribe of tribes of Israel is referred to, this is always the word that is used.
Gentiles, non-Israelites, are referred to as "ethnos". Ethnos is the word translated gentiles and nations:
Mat 28:19 Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:
Mat 10:5 These twelve Jesus sent forth, and commanded them, saying, Go not into the way of the gentiles, and into [any] city of the Samaritans enter ye not:
Luk 2:32 A light to lighten the Gentiles, and the glory of thy people Israel.
Luke 21:24 And they shall fall by the edge of the sword, and shall be led away captive into all nations: and Jerusalem shall be trodden down of the Gentiles, until the times of the Gentiles be fulfilled.
So why do you insist that Jesus meant "tribes of Israel" when he said "ethnos"? Presumably when this was put down in writing they knew the difference and used the proper language when they wished to convey "tribes of Israel" or gentiles. Do words mean anything?
Let's see, from the standard Koine Greek dictionary:
(1) a multitude (whether of men or of beasts) associated or living together,
(2) a company, troop, swarm
(3) a multitude of individuals of the same nature or genus
(4) a tribe, nation, people group
Transliterated it is "phyle" and it means: A tribe. a) in the NT all the persons descending from one of the twelve sons of the patriarch, Jacob 2) a nation, people
You have made my work easier. Obviously, ethnos and phyle“ both mean tribe and/or nation. They are interchangeable, and are often found translated interchangeably (cf Gen 25:16). This is because a tribe is synonimous with a nation (i.e. American Indian tribes are nations).
The Greek word phyle in the Old Testament is used in place of shebet, which means "clan." Clan is not a tribe. Clans subsist within a tribe as groups gathered around some idea, a party if you will.
Again, Christ specifically taught to avoid gentiles and Samaritans. Matthew 28:19 is a contradiction of his specific mission purpose and instructions given to his disciples (which they violated almost immediately after the Penetecost when they went to Samaria, probably out of desperation).
Any other interpretation makes no sense vis-a-vis Jesus and the OT. It makes perfect sense in the context of the religion created by Paul, which was precisely his agenda: to spread this Jewish sect and "sell" it to the pagan Greeks and Romans. In doing so, Paul saved Christianity from certain extinction in Israel. He repackaged it and sold it as a universal product.