Galatians 4:6 And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father.
7 Wherefore thou art no more a servant, but a son; and if a son, then an heir of God through Christ.
Every child of God was divinely given the Holy Spirit the moment he was adopted by God. The Spirit gives us an awareness that God is our Father. Only Jesus and Jesus’ followers can call the Father Abba. It is actually the Spirit of Jesus within us that cries out Abba. The Holy Spirit within us reveals the fact to us of our sonship. This Spirit of the Risen Christ within us opens our understanding to this. Our inheritance is in Christ and we are His inheritance as well. Paul is trying to remind us here about the benefits of grace of God over the law and that we receive sonship through the Spirit of Christ within us.