Paul contrasts two receptions here: one is of the spirit of slavery and the other is of the Spirit of adoption. We need to ask what Paul has in mind here. Here are some relevant questions. (1) Does ‘spirit’ in both uses refer to the Holy Spirit? (2) Who had the experience of slavery marked by dread? (3) How strong is the sense of having the Spirit of adoption?
It is possible that the term ‘spirit’ could refer once to the human spirit and once to the Holy Spirit or it could refer twice to the human spirit or it could refer twice to the Holy Spirit. We can see that Paul personalises the second reference and also distinguishes the ‘Spirit’ from believers, so the second reference clearly refers to the Holy Spirit.
The debate here has mainly concerned itself with the person and experience referred to by ‘spirit of slavery’. Some people think Paul is describing the period of conviction of sin when the Holy Spirit showed the Roman Christians that they were under its power. And Paul assures the Christians that they don’t receive the Holy Spirit in that manner now.
Another possible interpretation, which is connected in some ways to the one just mentioned, is that Paul is referring to the Spirit’s work in the old covenant as the time of spiritual bondage when God’s people did not possess the spiritual degree of knowledge as they now have.
In what way does the Spirit now come? Paul tells us that the Spirit enables believers to cry ‘Abba, Father!’ The word ‘cry’ points to a very strong cry, to a shout in their souls. We could say that the strength of the cry points to gladness, and that the use of ‘Abba’ points to a combination of intimacy and respect.
It has often been said that ‘Abba’ is what a Jewish child would call his father, often forgetting it is also how an adult Jewish child would address his father. There is no hint of childishness in spiritual things here. The point that Paul is making here is that the height of spiritual ecstasy, while remaining the norm for Christian living, is to address the great eternal God as Father.