Galatians 3  The true gospel, the Good news, is that, " All nations shall be blessed in you."  Paul says.  (God's promise to Abraham.) v8.   Abraham " believed God, and it was reckoned to him as righteousness". 
Abraham was saved by faith.  It is faith that brings salvation/justification.  It is faith that makes us into "sons of Abraham" and children of God.

Paul argues with those who want to add the requirements of the Law to the criterion for justification.

First of all,  not even the Jews who lived by the law could be justified by the Law.  ( 2:16)
All people, Jews or Gentiles, can only be justified by faith in Christ. (2:16-17)
If anyone could be justified, made righteous by the Law, then "Christ died needlessly". (v21)
The Spirit of God cannot be received by the Law, but only "by hearing with faith". ( 3:2)
Perfection, completion or holiness,  cannot come by the Law or flesh, but only by the Spirit.. through faith. ( v3)
Miracles... do not occur because of the works of keeping the Law,  but by "hearing with faith" (v5)
Abraham, himself, was "reckoned" righteous because of faith, not works.. the Law was not even given until 430 years later. (v8, 17)

The works of the Law don't - can't  justify anyone unless they keep ALL of them.  Since no one can do that, then all are "cursed".  But, Christ "redeemed us from the curse of the Law, having become a curse for us".  (v13) Thus the only way that we can be justified is by faith in Him.

Paul compares the Promise and the Law.
The Promise was given to Abraham and to his seed - to Jesus Christ.
The Law, which came 430 years later, was given because of "transgressions", through a mediator, for a limited time, until "the seed should come to whom the promise had been made" - until Christ came.
The Law can not impart life. (v21)
The Promise is fulfilled by faith in Jesus Christ.
The Law was useful as a tutor,  but it served its purpose only until faith came.
Through faith in Jesus Christ, we are "sons of God".  We "who were baptized into Christ" are
clothed.. with Christ....  We are made "one in Christ Jesus".  "And if you belong to Christ then you are Abraham's offspring, heirs according to the promise." 

"Heirs" - a sharer or possesser; one who receives a portion of a heritage.   In Romans 4:13 Paul wrote, "For the promise to Abraham or to his descendants that he would be heir of the world..."   Heirs of the "cosmos". The promise to Abraham was HUGE!
 Genesis 17: 4-8  gives us the exact promises that God made to Abraham:
He would father a multitude of nations.
He would be exceedingly fruitful.
He would have nations and kings come from him.
God would be his God and his descendant's God.
He would have the land of Canaan forever.
Then in Genesis 22:17-18, after Abraham obeyed God by offering Isaac as a sacrifice, God added this:
" I will greatly bless you and I will multiply your seed as the stars of the heavens and as the sand of the seashore; and your seed shall possess the gate of their enemies. And in your seed all the nations of the earth shall be blessed, because you have obeyed My voice." 
This is the inheritance promised to Abraham's descendants, which we are because we belong to Christ.   Our inheritance includes the nations, the blessings, the possession of the gates of the enemy, and most of all the fact that God, Elohim, the Creator and Sovereign Lord of all - is our God. Forever.

Jesus said, " Blessed are the gentle, for they shall inherit the earth." ( Matthew 5:5)  All that was promised to Abraham and to his descendants is ours in Christ Jesus.

Great are You Lord and highly to be praised.  You have given us amazing grace, that we should be Your children through Jesus Christ the Son of God, the Lamb who was slain for us. Thank You.
We have been blessed with so much more than we can understand or grasp... an inheritance for eternity..   We come to You with meek and humble hearts, confessing that we are unable to do anything to deserve Your mercy, and yet You have saved us by faith in Jesus Christ, who loved us and delivered Himself up for us.  Bless Your holy Name! Amen.