We lost our birthright through sin that is why we need an adoption:
(Old Testament | Isaiah 59:2)
2 But your iniquities have separated between you and your God, and your sins have hid his face from you, that he will not hear.
(New Testament | Romans 5:8 - 21)
8 But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us.
9 Much more then, being now justified by his blood, we shall be saved from wrath through him.
10 For if, when we were enemies, we were reconciled to God by the death of his Son, much more, being reconciled, we shall be saved by his life.
11 And not only so, but we also joy in God through our Lord Jesus Christ, by whom we have now received the atonement.
12 Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:
13 (For until the law sin was in the world: but sin is not imputed when there is no law.
14 Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam's transgression, who is the figure of him that was to come.
15 But not as the offence, so also is the free gift. For if through the offence of one many be dead, much more the grace of God, and the gift by grace, which is by one man, Jesus Christ, hath abounded unto many.
16 And not as it was by one that sinned, so is the gift: for the judgment was by one to condemnation, but the free gift is of many offences unto justification.
17 For if by one man's offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ.)
18 Therefore as by the offence of one judgment came upon all men to condemnation; even so by the righteousness of one the free gift came upon all men unto justification of life.
19 For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous.
20 Moreover the law entered, that the offence might abound. But where sin abounded, grace did much more abound:
21 That as sin hath reigned unto death, even so might grace reign through righteousness unto eternal life by Jesus Christ our Lord.
We are all His children:
(Old Testament | Psalms 82:6)
6 I have said, Ye are gods; and all of you are children of the most High.
(New Testament | Hebrews 12:6 - 9)
6 For whom the Lord loveth he chasteneth, and scourgeth every son whom he receiveth.
7 If ye endure chastening, God dealeth with you as with sons; for what son is he whom the father chasteneth not?
8 But if ye be without chastisement, whereof all are partakers, then are ye bastards, and not sons.
9 Furthermore we have had fathers of our flesh which corrected us, and we gave them reverence: shall we not much rather be in subjection unto the Father of spirits, and live?
