Why can believers cry 'Abba, Father'?
Paul writes in Romans 8:15 that believers have not received the spirit of bondage again to fear, but the Spirit of adoption, and by that Spirit they cry, Abba, Father. Because as many as are led by the Spirit of God are already the children of God.
Les explains from Romans 8:14-15 that Paul always writes to the believer, so this is for every true child of God, not just a few or the elite. Those led by the Spirit of God are already, not merely going to be, the sons or children of God. They have not received the spirit of bondage again to fear, but they have received the spirit of adoption, whereby they cry, Abba, Father. He also notes that the word adoption here does not mean what we normally think of, taking a child from some other union and legally making it ours.
For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father.