I like what Paul says in Romans 8:15, “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.’” What a transformation we experience when we trust in Jesus Christ - from fear to freedom in a powerful relationship of adoption into sonship.
F.F. Bruce suggests not to see things through our contemporary cultural understanding, rather than the Greco-Roman culture of Pauls’ day. He writes: “The term ‘adoption’ may have a somewhat artificial sound in our ears; but in the Roman world of the first century AD an adopted son was a son deliberately chosen by his adoptive father to perpetuate his name and inherit his estate; he was…not in the smallest degree inferior in status to a son born in the ordinary course of nature, and might well enjoy the father’s affection more fully and reproduce the father’s character more worthily.”
It’s not clear if receiving the Spirit of sonship causes us to say, “Abba,Father.” or when we cry out, “Abba, Father,” the Spirit bears testimony with our spirit that we are God’s children. Basically it doesn’t matter. Once we’ve received Jesus Christ it is proper for us to call God our Father, and say “Abba, Father” as an expression of intimacy and warmth, like a child would express to their loving father.
Try visualizing yourself sitting on the lap of God the Father, embraced by Him. Then express your heart felt gratitude for His love.