We are instructed to call God our Father, revealing the kind, gentle, loving nature of our God. The term "Abba" is an affectionate term that a young child calls his father, which is like our term "daddy." It is a term used for fondness. It removes the idea of God as a strict judge and carries the image of Him as a loving Father who cares, understands, and is our best friend. "Behold what manner of love the Father hath bestowed upon us, that we should be called the sons of God" (1 John 3:1).
We are sons of God by adoption. Jesus was the Son of God by nature. As Jesus said to the Jews, ye are of your Father the devil (John 8:44; Ephesians 2:3).
Jesus purchased us and made us adopted sons of God.
We are not just heirs, we are joint-heirs with Christ. To think that we share equally with the one who has inherited everything God is, and has, is beyond our imagination.