5 So again I ask, does God give you his Spirit and work miracles among you by the works of the law, or by your believing what you heard? 6 So also Abraham “believed God, and it was credited to him as righteousness.” 7 Understand, then, that those who have faith are children of Abraham. 8 Scripture foresaw that God would justify the Gentiles by faith, and announced the gospel in advance to Abraham: “All nations will be blessed through you.” 9 So those who rely on faith are blessed along with Abraham, the man of faith. (Galatians 3:5-9)
In Genesis 12:1–3 God gave to Abraham a promise of His continued blessing on him and all who came after him who would choose to believe in Him, the God of the Bible. As a result, Abraham and those who would believe were given a mission to "be a blessing" to all the nations. This mission included telling others of God, of His salvation and His goodness.
Now, in Genesis 17:4-5 God spells out the terms of his covenant with Abraham: "Behold, my covenant is with you, and you shall be the father of a multitude of nations. No longer shall your name be Abram, but your name shall be Abraham, for I have made you the father of a multitude of nations." The word "multitude" includes "all the nations of the earth." So, from the beginning, God hasn't only promised His salvation to the believing Jews, but also to those non-Jews who would believe.
The blessing includes justification, which means that in spite of all our sins, God counts us to be righteous through His Son. Romans 5:18-19 sums it up well: “Consequently, just as the result of one trespass was condemnation for all men, so also the result of one act of righteousness was justification that brings life for all men. For just as through the disobedience of the one man the many were made sinners, so also through the obedience of the one man the many will be made righteous.”
"To be justified means more than to be declared "not guilty." It actually means to be declared righteous before God. It means God has imputed or charged the guilt of our sin to His Son, Jesus Christ, and has imputed or credited Christ's righteousness to us."
- Jerry Bridges
The blessing also includes regeneration by the Spirit of God who seals us for eternal life. Galatians 3:14 says that Christ became a curse for us "that in Christ Jesus the blessing of Abraham might come upon the Gentiles, in order that we might receive the promise of the Spirit through faith."
"The indwelling Spirit shall teach him what is of God and what is not. This is why sometimes we can conjure up no logical reason for opposing a certain teaching, yet in the very depth of our being arises a resistance."
-Watchman Nee
