Apostle Paul says the "gospel of Christ is God's power unto salvation for everyone who believes, for the Jew first and also for the Greek" (Rom. 1:16).
The "gospel of Christ" is simply the "teaching or revelation" of Christ concerning God's PLAN of salvation. After all, God "sent Christ into the world that the world through him might be saved" (John 3:17).
Clearly, what God promised Abraham, Jacob and Isaac is not pertinent to how one may enter the kingdom of heaven and attain eternal life.
What matters is what Jesus says in Matt. 5:20 to wit: "For I say to you, that UNLESS your righteousness EXCEEDS the righteousness of the scribes and Pharisees, you will by no means enter the kingdom of heaven."
Therefore, the pertinent question to ask yourself is, "How can MY righteousness EXCEED the righteousness of the scribes and Pharisees?"
How were the scribes and Pharisees "righteous" before the eyes of God? How can YOU be "righteous" before the eyes of God.
Apostle John wrote that the "advocate to the Father is Jesus Christ, the righteous" (1 John 2:1). How was Christ "righteous" before the eyes of God?
The scribes and Pharisees were "righteous" by diligently observing the law of Moses and annual offering of blood by the high priest for their sins to be forgiven ANNUALLY.
But according to Jesus, this does not warrant anyone's entering the kingdom of heaven. One needs to EXCEED the "righteousness" of the scribes and Pharisees to enter the kingdom of heaven.
How can one EXCEED the righteousness of the scribes and Pharisees?
Jesus is "righteous" because he has no sin. He was "sanctified" by the Father (John 10:36). In other words, Jesus was "made righteous" by the Father. Naturally, the "righteousness" of Jesus EXCEEDS the "righteousness of the scribes and Pharisees.
Therefore, for one to EXCEED the righteousness of the scribes and Pharisees in order to enter the kingdom of heaven, one's sins have to be forgiven and be RECONCILED to God.