Gal 4:6-7, 6 And because you are sons, God has sent forth the Spirit of His Son into your hearts, crying out, “Abba, Father!” 7 Therefore you are no longer a slave but a son, and if a son, then an heir of God through Christ
Inheritance is reserved for heirs. In God’s kingdom, an heir must be a son. A slave under bondage need redemption in order to secure freedom and change of nature. Paul has shown that those under the law are in bondage. Before salvation, our nature as slaves disqualified us. From this bondage Christ came to redeem us that is, buy us back from the slave market of sin and make us sons by adoption.
God rich in grace adopted us into His family and freely bestowed His inheritance on us. He in addition became our inheritance and portion. The Spirit of God’s only Begotten Son is in our hearts leads us to cry Abba Father. He attests our sonship by adoption. By faith we are one with the Son, so that what is His is ours. His Sonship ensures our sonship; His Spirit ensures our share in the same. As heirs, we share in all that are are His; His love, grace, mercy, wisdom, power, truth, faithfulness etc.