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?
.... 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.
Righteousness is not really much in the doing but in the heart. Romans 10:10 says, "For with the HEART man BELIEVETH UNTO RIGHTEOUSNESS; and with the mouth confession is made unto salvation." and Romans 9:30 says "What shall we say then? That the Gentiles, which followed not after righteousness, have attained to righteousness, even the righteousness WHICH IS OF FAITH."
Scriptures speaks about what righteousness the scribes and Pharisees have:
Matthew 23 [CAPS MINE]
1 Then spake Jesus to the multitude, and to his disciples,
2 Saying The SCRIBES and the PHARISEES sit in Moses' seat:
3 All therefore whatsoever THEY bid you observe, that observe and do; but do not ye after their works: for THEY SAY, AND DO NOT.
4 For THEY bind heavy burdens and grievous to be borne, and lay them on men's shoulders; but THEY themselves will not move them with one of their fingers.
5 But ALL THEIR WORKS THEY DO for TO BE SEEN OF MEN: they make broad their phylacteries, and enlarge the borders of their garments,
13 But woe unto YOU, SCRIBES and PHARISEES, HYPCRITES! for ye shut up the kingdom of heaven against men: for ye neither go in yourselves, neither suffer ye them that are entering to go in.
14 Woe unto YOU, SCRIBES and PHARISEES, HYPCRITES! for ye devour widows' houses, and for a pretence make long prayer: therefore ye shall receive the greater damnation.
15 Woe unto YOU, SCRIBES and PHARISEES, HYPCRITES! for ye compass sea and land to make one proselyte, and when he is made, ye make him twofold more the child of hell than yourselves.
16 Woe unto YOU, YE BLIND GUIDES, which say, Whosoever shall swear by the temple, it is nothing; but whosoever shall swear by the gold of the temple, he is a debtor!
17 YE FOOLS and BLIND: for whether is greater, the gold, or the temple that sanctifieth the gold?
23 Woe unto YOU, SCRIBES and PHARISEES, HYPCRITES! for ye pay tithe of mint and anise and cummin, and have omitted the weightier matters of the law, judgment, mercy, and faith: these ought ye to have done, and not to leave the other undone.
24 YE BLIND GUIDES, which strain at a gnat, and swallow a camel.
25 Woe unto YOU, SCRIBES and PHARISEES, HYPCRITES! for ye make clean the outside of the cup and of the platter, but within they are full of extortion and excess.
26 THOU BLIND PHARISEE, cleanse first that which is within the cup and platter, that the outside of them may be clean also.
27 Woe unto YOU, SCRIBES and PHARISEES, HYPCRITES! for ye are like unto whited sepulchres, which indeed appear beautiful outward, but are within full of dead men's bones, and of all uncleanness.
28 Even so YE also OUTWARDLY APPEAR RIGHTEOUS UNTO MEN, BUT WITHIN YE ARE FULL OF HYPOCRISY AND INIQUITY.
29 Woe unto YOU, SCRIBES and PHARISEES, HYPCRITES! because ye build the tombs of the prophets, and garnish the sepulchres of the righteous,
33 YE SERPENTS, YE generation of VIPERS, how can ye escape the damnation of hell?
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.
Believe and have faith in God, even in His Son Jesus Christ.
Philippians 3:9 And be found in him, not having mine own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith: