Abba is the Aramaic word a child used for a beloved father — warm, intimate, and trusting, closer to dear father than to the formal father. It was the everyday word of the household, and its use for God was startling. Jesus prayed it in Gethsemane — Abba, Father, all things are possible unto thee — addressing the Almighty with the intimacy of a son.
The astonishing gift of the gospel is that believers are invited to use the same word. Paul writes that we have received the Spirit of adoption, whereby we cry, Abba, Father — and again, that God has sent the Spirit of his Son into our hearts, crying, Abba, Father. To pray Abba is to know oneself not as a servant but as a beloved child, brought into the family of God.
Abba in Scripture
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.