The first book in the Bible tells us who and what Israel is. See Genesis 32:28.
Genesis 32:28 Then the man said, “Your name will no longer be Jacob, but Israel, because you have struggled with God and with men and have overcome.”
Did you see that? The one who struggles with God and with men and overcomes…
And Jesus in the last book of the Bible confirms this message. See these scriptures in Revelation. Read what Jesus says about overcoming.
Revelation 2:7 He who has an ear, let him hear what the Spirit says to the churches. To him who overcomes, I will give the right to eat from the tree of life, which is in the paradise of God.
Revelation 2:11 He who has an ear, let him hear what the Spirit says to the churches. He who overcomes will not be hurt at all by the second death.
Revelation 2:17 He who has an ear, let him hear what the Spirit says to the churches. To him who overcomes, I will give some of the hidden manna. I will also give him a white stone with a new name written on it, known only to him who receives it.
Revelation 2:26 To him who overcomes and does my will to the end, I will give authority over the nations–
Revelation 3:5 He who overcomes will, like them, be dressed in white. I will never blot out his name from the book of life, but will acknowledge his name before my Father and his angels.
Revelation 3:12 Him who overcomes I will make a pillar in the temple of my God. Never again will he leave it. I will write on him the name of my God and the name of the city of my God, the new Jerusalem, which is coming down out of heaven from my God; and I will also write on him my new name.
Revelation 3:21 To him who overcomes, I will give the right to sit with me on my throne, just as I overcame and sat down with my Father on his throne.
Revelation 21:7 He who overcomes will inherit all this, and I will be his God and he will be my son.
And there is only one way a person can overcome this world, and that is through Jesus Christ our Savior.
1 John 5:5 Who is it that overcomes the world? Only he who believes that Jesus is the Son of God.
Therefore, who and what is Israel---Israel is people who overcome by faith in Jesus Christ.
Just as Jacob is called Israel by overcoming, we too are Israel by overcoming by faith in Jesus.