I think that as Paul wrote this, he probably looked back at his words on Romans 6, and felt he needed to make some clarification. In Romans 6, he talked about how we used to be slaves to sin, but now we are slaves to God. It seems a strange concept to be a slave to God. While on one hand, it does carry the idea that we serve God and are wholly his, which I think was Paul’s point, it nevertheless also carries the idea of no freedom and fear of punishment.
And so I think Paul seeks to clear up those possible misconceptions in these verses. He says,
For you did not receive a spirit that makes you a slave again to fear, but you received the Spirit of sonship. And by him we cry, “”Abba,” Father.” The Spirit himself testifies with our spirit that we are God’s children. Now if we are children, then we are heirs–heirs of God and co-heirs with Christ, if indeed we share in his sufferings in order that we may also share in his glory. (15-17)
In other words, as the Spirit leads you, he’s not a cruel taskmaster that brutalizes you for your failures and mistakes. He’s not someone that insists that we are no good, and totally unacceptable to God. Instead, when we are discouraged because of our failures, and feel, like the prodigal son did, that there’s no way we can still call God “Father,” the Spirit whispers, “Hey! Listen to me! You are still God’s child and he still loves you. It’s okay for you to call him, ‘Father.’ It warms his heart to hear you call him that.”
And as we go through suffering, the Spirit reminds us that there is hope for the future. That we are God’s heirs, and that our suffering will not last forever.
In short, we are much more than mere slaves of God. We are his beloved children. May we never forget that.