First, Paul tries to claim the promise of Genesis 15:6 created faith alone as a basis of Abraham being right with God. Gal. 3:6. (As discussed elsewhere, this verse had nothing to do with imputed righteousness.) Then Paul says we inherit the promise of Genesis 15:5 (blessing of offspring as the number of stars) by the sheer step of faith that supposedly justified Abraham in Genesis 15:6. (Gal. 3:26.) The problem is that Paul has utterly ignored that there is no covenant offered with either Abraham or with any offspring in Genesis 15:5-6. A covenant with Abraham is only offered in Genesis 17:1-7, specifically mentioning it runs in favor of Abraham's offspring: "And I will establish my covenant between me and thee and thy seed after thee in their generations for an everlasting covenant, to be a God unto thee, and to thy seed after thee." (Gen 17:7.) Please also note it was an eternal covenant. Its terms would never expire.
Furthermore, the condition of the Abrahamic covenant is not faith, but the obedience of Abraham: "walk before me, and be thou blameless and I will make my covenant between me and thee, and will multiply thee exceedingly." (Gen 17:1-2.) Consequently, Abraham and his "seed after you" had a duty in turn to "keep my covenant." (Gen. 17:3.) God repeats this in Genesis 18:19, saying that the covenant promise of God is contingent on Abraham obeying and teaching his children to obey God's commands. Then, after Abraham's death, God affirmed to Isaac that Abraham had obeyed all God's law, which now justified God keeping His side of the covenant to Abraham's seed, namely Isaac:
(1)... And Isaac went... unto Gerar. (2) And Jehovah appeared unto him, and said, Go not down into Egypt. Dwell in the land which I shall tell thee of. (3) Sojourn in this land, and I will be with thee, and will bless thee. For unto thee, and unto thy seed, I will give all these lands, and I will establish the oath which I sware unto Abraham thy father. (4) And I will multiply thy seed as the stars of heaven, and will give unto thy seed all these lands. And in thy seed shall all the nations of the earth be blessed. (5) Because that Abraham obeyed my voice, and kept my charge, my commandments, my statutes, and my laws. Gen 26:1-5 (ASV)
Thus, Paul turned a mere promise to Abraham alone in Genesis 15:5-6 into something it was never intended to be. Paul made it a distinct covenant of righteousness by faith that belonged to Abraham's offspring. Paul has caused generations of Christians to ignore the true covenant made between Abraham and God that was inheritable. It was clearly one premised on Abraham "walking with me blamelessly" in which case God "will" later enter into a covenant with Abraham. Gen. 17:1-2. We know from Genesis 26:1-5 that Abraham did obey God's commandments, and that God did assume His obligation under the covenant. God at that juncture brought a blessing on offspring of Abraham. The blessing was obtained by a covenant of obedience, not one upon faith alone.
Indeed, thereafter the terms of the true Abrahamic Covenant remained "eternal," just as God said. (Gen. 17:7.) This is ignored by Pauline Christians for it would overthrow Paul's Gospel if ever accepted.
For the same terms of the Abrahamic Covenant were then repeated by Moses in the Law in Deuteronomy 6:25: "And it shall be righteousness unto us, if we observe to do all this commandment before Jehovah our God, as he hath commanded us." Mercy is always possible for transgression if you turn from evil (Deut.13:17), but righteousness was only to be imputed again if repentance ensued. (Ezekiel 33:12-14.)
This is why the Prophet Daniel likewise repeated the obedience-requirement for covenant promises to be kept: "And I prayed unto Jehovah my God, and made confession, and said, Oh, Lord, the great and dreadful God, who keepeth covenant and lovingkindness with them that love him and keep his commandments..." Dan 9:4.
Then of course Jesus put the same emphasis on "obeying" all "my teaching" for eternal life in John 8:51. Then, of course, if you fail to obey but you repent from sin and turn back to obedience, you are once more "justified" with God. (Luke 18:10 ff.) But those who "keep on disobeying the son continue to have the wrath of God reside on them." (John 3:36 ASV.)
Thus, we see all three covenants have identical principles on obedience and justification. They are a continuum of an identical message. God's promise of salvation is upon those who obey all His commands, statutes and ordinances. Atonement under such a system only applied to those who fled the altar first to be reconciled to the one they knew they had sinned against, as Jesus Himself said. (Matthew 5:23-24.) As Jesus repeatedly said, if you violate the commandments, you must engage in severe repentance (figuratively cut off offending body parts) to avoid being sent to hell. (Matthew 5:29,Matthew 18:8, and Mark 9:42-48.) Thus, all three covenants match each other with the same salvation doctrine.
This explains why God could say the Abrahamic covenant was "an everlasting covenant" (Gen 17:7) just as God could say "these ordinances" given Moses in the Law shall be "everlasting for all generations." (Ex. 27:21; 30:21; Lev. 6:18; 7:36; 10:9; 17:7; 23:14, 21, 41; 24:3; Num. 10:8.)
But if obedience to the Law creates justification (Deut. 6:25) this never meant it does so without faith, as Paul assumed. A command to have faith is not absent in the Law. Jesus said the "weightier matters of the Law" include not only Justice and Mercy, but also Faith. (Matt. 23:23.) A command to have faith/trust is found numerous times in the Law and Prophets, e.g., Deut. 31:6; Isaiah 26:4; and Jer. 17:7.