Even before God created the world his purpose for us was adoption. And when the set time was fully come God acted to fulfil his purpose by sending his Son. (Galatians 4.4-5). The Father and Son enjoyed love relationship with one another before they made the world. Out of this love relationship, the Father sent his Son into the world as human. He sent him so that we might be adopted as sons (the term sons includes daughters). Jesus is the Father’s unique Son and we become sons and daughters because of our union with The Son.
Because you are his sons, God sent the Spirit of his Son into our hearts, the Spirit who calls out, “Abba, Father (Galatians 4: 6). The Son was sent into the world to make us sons. Now the Spirit of that same Son is sent into our hearts crying Abba Father. Abba is the Aramaic word for Father. While God’s Son was on earth, he addressed God as Abba (Mark 14:36). And now, through the Spirit of God’s Son, the Son’s own Abba Father cries out within us as we share in the Son’s relationship with Abba. In heaven, Jesus speaks his own Abba before the Father and his Abba is echoed in us through the Spirit of the Son within us.
Paul is speaking here about sharing in the Son’s love relationship with Abba Father together. All the pronouns are plural. We are Sons and daughters communally. And so, we cry Abba Father together. As the Son relates in love to the Father, we are all caught up in this together in the Spirit.