15:1

I am the true vine

Over and over in the Old Testament Israel is pictured as the vine or the Vineyard of God.
the vineyard of the Lord is the house of Israel (Isaiah 5:1-7)
Yet I planted you a choice vine (Jeremiah 2:21)
Ezekiel 15 likens Israel to the vine as does Ezekiel 19:10.
Israel is a luxuriant vine (Hosea 10:1)
Thou didst bring a vine out of Egypt (Psalm 80:8)

The vine had become a symbol of the nation of Israel. It was the emblem on the coins of the Maccabees. The vine was part and parcel of Jewish imagery, and the very symbol of Israel.

Yet Jesus proclaims that he is the true or real vine. It is as if he is saying "you think that because you belong to the nation of Israel you are a branch of the true vine of God, but the fact that you are a Jew can not save you. the only thing that can save you is to have an intimate living fellowship with me, for I am the vine of God and you must be branches joined to me.

the Gospel of John volume 2, William Barclay p 172-173