Galatians 4:6 Because you are his sons, God sent the Spirit of his Son into our hearts, the Spirit who calls out, “Abba, Father.” So you are no longer a slave, but a son, and if a son, then an heir through God.
Jeremiah 3:19 “‘I said, How I would set you among my sons, and give you a pleasant land, a heritage most beautiful of all nations. And I thought you would call me, My Father, and would not turn from following me.
Romans 8:9 You, however, are not in the flesh but in the Spirit, if in fact the Spirit of God dwells in you. Anyone who does not have the Spirit of Christ does not belong to him.
Jeremiah 32:38-41 And they shall be my people, and I will be their God. I will give them one heart and one way, that they may fear me forever, for their own good and the good of their children after them. I will make with them an everlasting covenant, that I will not turn away from doing good to them. And I will put the fear of me in their hearts, that they may not turn from me. I will rejoice in doing them good, and I will plant them in this land in faithfulness, with all my heart and all my soul
When we groaned under a sense of sin, and were shut up by it as in a prison, we feared that the law would punish us for our iniquity, and our life was made bitter with fear. Moreover, we strove in our own blind self-sufficient manner to keep that law, and this brought us into yet another bondage, which became harder and harder as failure succeeded to failure: we sinned and stumbled more and more to our soul’s confusion. But now that faith has come we see the law fulfilled in Christ, and ourselves justified and accepted in him: this changes the slave into a child, and duty into choice. Now we delight in the law, and by the power of the Spirit we walk in holiness to the glory of God. Thus it is that by believing in Christ Jesus we escape from Moses, the taskmaster, and come to Jesus, the Saviour; we cease to regard God as an angry Judge and view him as our loving Father. The system of merit and command, and punishment and fear, has given way to the rule of grace, gratitude, and love, and this new principle of government is one of the grand privileges of the children of God
Now, faith is the mark of sonship in all who have it, whoever they may be, for “you are all the children of God by faith in Christ Jesus Gal. 3:26). If you are believing in Jesus, whether you are Jew or Gentile, bond or free, you are a son of God. If you have only believed in Christ of late, and have but for the past few weeks been able to rest in his great salvation, yet, beloved, now are you a child of God. It is not an after privilege, granted to assurance or growth in grace; it is an early blessing, and belongs to him who has the smallest degree of faith, and is no more than a babe in grace. If a man be a believer in Jesus Christ his name is in the register-book of the great family above, “for you are all the children of God by faith in Christ Jesus.” But if you have no faith, no matter what zeal, no matter what works, no matter what knowledge, no matter what pretensions to holiness you may possess, you are nothing, and your religion is vain. Without faith in Christ you are as sounding brass and a tinkling cymbal, for without faith it is impossible to please God. Faith then, wherever it is found, is the infallible token of a child of God, and its absence is fatal to the claim. (Spurgeon)