'Rightly Dividing the Word'
Wickedness of Men
Gen 6:5 Then the Lord saw that the wickedness of man was great in the earth, and that every intent of the thoughts of his heart was only evil continually.
Since the fall of Adam, men had become more and more wicked until only eight souls were left at the deluge.
After the deluge, the descendants of the righteous Noah were once again becoming more and more wicked. Men failed the Noahic Covenant miserably by becoming idolaters until there were very few god-fearing men left.
Abrahamic Covenant
Gen 12:1-2 Now the Lord had said to Abram: "Get out of your country, from your family and from your father's house, to a land that I will show you. I will make you a great nation; I will bless you and make your name great; and you shall be a blessing."
God found Abraham, who worshipped the Almighty God, amongst the idolaters. Immediately, He call him out to dwell in a foreign land and He made the covenant of circumcision (including monotheism) with him and his descendants .
So all the descendants of Abraham must keep the Noahic law and the new Abrahamic law. Descendants of Abraham includes Ishmael, Isaac, Edom, and Jacob.
Mosaic Covenant
Deu 5:1-2 And Moses called all Israel, and said to them: "Hear, O Israel, the statutes and judgments which I speak in your hearing today, that you may learn them and be careful to observe them. The Lord our God made a covenant with us in Horeb.
As the descendants of Jacob increased, God sent all the 70 Israelites to dwell in Egypt to preserve them from idolatry (for the Egyptians would not eat with them nor allowed them to worship in their temple). They were slaves for almost 400 years until God sent Moses to deliver them from the Egyptians.
With the failure of the Noahic covenant, God decided to live amongst the Israelites in the Mosaic Covenant.
To strengthen their faith, God allowed them to see the pillar of cloud in the day and the pillar of fire at night above the tabernacle.
So every Israelite must observe the Noahic law, the Abrahamic law and the new Mosaic law, which had included the Noahic and Abrahamic laws.
Israelites, who observed the Mosaic Covenant commonly known as Judaism, were called Jews.
Note: Gentiles are non Jews. They only need to observe the Noahic law and they have nothing to do with the Mosaic law.
Failure of the Jews
2 Chr 24:19 Yet He sent prophets to them, to bring them back to the Lord; and they testified against them, but they would not listen.
Having seen the great display of God's amazing power in their deliverance from Egypt and God's presence dwelling amongst them, yet the Israelites had no fear of God.
They were more and more wicked and became hypocrites. They even defiled the temple of God by worshipping idols inside the temple.
God had to remove all these abominations and wickedness by sending their enemies to destroy Jerusalem and the temple, and to send the Israelites into exile.
During this period of distress, God comforted them that He would give them a new heart and a new spirit to dwell within them in the New Covenant.
Eze 11:19 Then I will give them one heart, and I will put a new spirit within them, and take the stony heart out of their flesh, and give them a heart of flesh,
