Sin and death entered this world through the first man, Adam. That fall affected all humanity to come afterward, not just one people group, not Israel only.
God was grieved with the sinfulness of man and wiped them out in a flood, saving only Noah and his family, from whom the world would be repopulated (Genesis 10). Noah was not Israel only.
God was again grieved by the rebelliousness of man at Babel and so He divorced Himself from, and scattered, the nations (humanity torn apart) and placed them under the authority of heavenly beings, the sons of God (Deuteronomy 32:8). Out from the nations God called Abraham, with whom He cut a covenant; a covenant which would lead to the blessing of all nations (Genesis 15-18); a reconciliation with God. Abraham was not Israel only.
God singled out Israel (Deuteronomy 32:9-14), to be a nation of priests to the nations, to build a dwelling place for God so that all could come to worship and pray and know the God of Israel (1 Kings 8:41-43). Israel was not for Israel only.
Israel broke their covenant with God (Deuteronomy 32:15-30) and they did not keep the law God had given to them, and so they were under a curse (Deuteronomy 28:15-68). The curse scattered Israel in exile (Israel torn apart), but God promised a Redeemer. But, that Redeemer was not for Israel only (Isaiah 49:5-6).
Jesus came to the lost sheep of Israel (Matthew 15:24) to restore them to their position of priests to the nations; to bring back together that which was torn apart. Jesus saved them from the curse of the law (Galatians 3:13). To save them, He became them (Matthew 2:15). In becoming them, Jesus took on their vocation (Galatians 3:14): to be a priest to the nations (Hebrews 4:14), reconciling the world to God; bringing back together that which had been torn apart (2 Corinthians 5:16-19).
Jesus is the true Israel. Jesus is the only Israel. Jesus is the saviour of all mankind.
Not Israel only.
Related reading: Some Speculations on Romans 5; The Incompetent Crankery of the Israel Only Movement
Pursuit of Percipience 