While we started off as slaves, we have become actual heirs through Christ’s sacrifice. Verse 7 puts it very simply, So you are no longer a slave, but a son; and since you are a son, God has made you also an heir. According to the notes in my Bible, under Roman law, an adopted child was guaranteed all legal rights to his Father’s property, even if he was a former slave, he was equal to the other sons. By being adopted by God through His son we are full heirs.