And because you are sons, God has sent the Spirit of his Son into our hearts, crying "Abba! Father!" Paul can rightly declare his addressees as sons because of their baptisms into the church. He is speaking the liturgy and returns them to God’s act in them. In this way the liberation from the enslaving cosmos is a returning to baptism which is the beginning and ending of God’s act on the cross. Only through Holy Baptism which truly liberates, crucifies, recreates, and strengthens can the son truly be a son and cry out the Lord’s Prayer before the community of the faithful. The filioque is set forth as Jesus shares in the full deity the Father, sending His spirit into human hearts claiming them as his own. The full communicative union is now complete as the Father sees all humanity incorporated into his Son, whom he loves and finds pleasing. By the crucifixion in baptism, the new creation can come out from her hiding place, awaking from a long nightmare whose peaceful end had been promised since the beginning.
So you are no longer a slave but a son, and if a son, than an heir through God. This is Paul’s magnum opus in which he tears apart the erroneous and limited promise likely espoused by the Judaizers for claming Abraham and following in the law. The false teachers have made a mockery of God’s work, stripped Christ of his glory, and pointed back to the enslaving principles of the supernatural and demonic power of the law. Paul is preaching the communion the Galatians already have in God through Jesus Christ. Though God has always been present among his people he has now performed the act which is the only act that matters.
The crucifixion and resurrection given to the church is itself the incarnation in which God heals every wound and calms every terror. ‘God with us’ takes on a meaning that only Luther can rightly express that Christ is “with us in the muck and work of our lives so much that his skin smokes” (WA 4:608.32 and 609:1). This singular cosmic act rectifies everything that has gone awry and violently wages war against every enslaving power – law, flesh, and idolatry. Finding sonship in God and His inheritance for Paul is composed in the person of Jesus whose greatest invitation is love toward creation. Being an heir to God is receiving Him in such a way that orients humanity toward and organically into one another, as Jesus immerses himself into his church.