Abraham as faithful to his creator that God loved him so much. Now God inorder to show his love for him gave him a covenant that the whole nation will be blessed thru his seed.
Gn 22:18 And in thy seed shall all the nations of the earth be blessed; because thou hast obeyed my voice.
Now God was not mentioning abt Isaac, but Jesus Christ, who was a descendant of Abraham in flesh.
Ga 3:16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
Till the messiah come, God ruled his people thru the law that he gave. And made the people realised how important it was for them to have a messiah who can save them from all the mess. Now they were not able to fulfill the law as God commanded. Prophets prophesised abt the coming of the Messiah and abt the new covenant God is going to have with humans.
Zch 8:8 And I will bring them, and they shall dwell in the midst of Jerusalem: and they shall be my people, and I will be their God, in truth and in righteousness.
The appointed time came, for the messiah to save his people from the sin. Mary gave birth to Jesus. He grew , full in holy spirit.Knowing that he is going to be the passover lamb for the whole world he lived 33 years in this world and gave himself as a attonment for the sins of us. While he was alive he taught us how to do the will of the father (sermon on the mount), what to do and what not to do
Jesus fulfilled the promise given to Abraham by dying on the cross. He resurrected on the 3rd day which gave us the faith in resurrection. and sat on the right hand side of God. The new covenant was established which zechariah prophesied...holy spirit is given to us. Salvation is the main purpose of the Gospel. We are saved from the world of sin, from the carnal mind, from the lustfull flesh, from unrighteousness etc....
Death was abolished by him. It is true we still die but Jesus won victory over death, by giving us eternal life after death.
Gospel tells us abt the love of God and his son. We being sinners he came for us sinners.
Ro 5:8 But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us.
That is the love that the gospel spreads. the love of his son, who was god/ in the form of God...not for his reason, but for sinners like us became in the form of man, took all the pain for no reason of his, for us.
Phl 2:6 Who, being in the form of God, thought it not robbery to be equal with God:
Phl 2:7 But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men:
Phl 2:8 And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross.
The law was fulfilled by Jesus christ and we no longer are under the law. Now it is not the work or rituals that will save us, but just believe in Jesus Christ, in his resurrection and do the will of God. U are saved. It is a curse for us if we being under the law do not fulfill it. but now we are free. We are under the "law of liberty". Have faith in God and wait to see Jesus who saved us coming for the second time to take us to his kingdom.