In Romans 8:15-17, Paul writes to the Christians in Rome,

You have not received a spirit of servile fear again, but rather you have received a spirit of adoption, in which you cry out, ‘Abba, Father’. The spirit itself testifies to our spirit that we are God’s children. And if we are God’s children, then we are also heirs, heirs of God and co-heirs with Christ.

Abiel Abbot Livermore comments on this text,

15. The Christian believer, instead of being admitted to a less intimate relationship with God than was enjoyed by the chosen people, or being subjected by the new dispensation to a slavish fear, as was the case, necessarily, with the Jews, because they were a rude people and their faith was a rudimental one, entered into the most delightful spirit of adoption, or filiation with God, so that he could without blame or fear call God by his most endeared name of Father. Olshausen remarks, that “Abba, like Papa, can be spoken by the mouth even of the babbling child, and properly therefore characterizes genuine childlike disposition and manner.” Gal. iii. 26; iv. 6. The rise and progress of true religion in the soul may be tested very much by this criterion, how much we have of the true filial and trusting love of a child towards our Heavenly Father, and how far we can, under all circumstances, even of darkness and grief and fear, cast ourselves into his arms, and say, “Even so, Father, for so it seemed good in thy sight.” 16, 17. Paul now comes to the proof that this spirit of adoption was a reality, and not a fancy, and he appeals to the inward testimony of the spirit. The disposition itself is the proof, and carries its own weight of argument with it. The voice of God in the heart cannot deceive us, but gives us persuasive evidence that we are the sons and daughters of an Almighty Parent. But an important inference follows, that, if we are the children of God, then, as in earthly relationships of a like kind, we are his heirs, and have an inheritance, in prospect, great and glorious as becomes such a testator, and one too which we share with the elder brother and oldest son of the spiritual family, our Savior Jesus Christ. 2 Cor. i. 22; 1 John iv. 13. What delightful assurances, what comforting hopes, and what animating motives are supplied from this source to enable us both to do and to endure all the holy will of Him whose children we are! We may look up to heaven, and forward to eternity, if this conviction be planted deep and strong in our heart of hearts, without distrust or terror. For we know that here and hereafter, now and forever, all is eventually well, all is right, all is good, all is infinitely blessed and glorious, in the universe of One so mighty, so wise, and so good. Luke xxii. 29; John xvii. 24; Rev. iii. 21.