Israel as the firstborn

Exodus 4:22 And thou shalt say unto Pharaoh, Thus saith the LORD, Israel [is] my son, [even] my firstborn:

God openly declared Israel as his firstborn children in the Bible. Kindly note that this does not mean that they are the only children of God but they were the first nation to be adopted as God's children for salvation and others were to follow. 

God's plan of salvation can be traced back to Abraham (although I don't have space and time to explain it since the fall of Adam) whom he called out of worshipping idols unto worshipping the one true God. Remember that wickedness before Abraham was too much in Noah's time that God sent a flood that destroyed the whole world except eight people who were in the Ark. God regretted for having done such an act and decided to draw man through love and grace. So he began a search of someone through whom He could preserve a righteous line for the coming saviour.

Through Abraham God brought forth the nation of Israel who are usually called Jews or Hebrews. When Abraham believed God, even though a sinner, he was counted righteous in the same way that we are made righteous. He is called the father of believers or those who put faith in the God of heaven for salvation without works. The rest of the world termed as gentiles would hear about God through them. But not all Israel believed in God's plan of salvation just like not many gentiles do not believe. That is why they crucified the saviour Jesus. But those who believe become God's children by faith, what Paul calls a true Jew as one who is inward and not outwardly by descent or through the natural line of Abraham.

I am a scientist and scientist does experiments. But if a vaccine is to be rolled out to the world, it is sampled among a certain group of the population called a sample. They are the first to receive the dose to prove whether it works before the rest get it. 

God, in heaven looks towards the earth seeking a man whom He could use. You may be the first to believe and be called out in your family, tribe, region or workplace but through you He will call the rest to Himself, hallelujah.

Further scriptures 

Romans 2:28-29 For he is not a Jew, which is one outwardly; neither [is that] circumcision, which is outward in the flesh: But he [is] a Jew, which is one inwardly; and circumcision [is that] of the heart, in the spirit, [and] not in the letter; whose praise [is] not of men, but of God.