Believers are the Sons of God even before they believe in Christ, in the same sense that they are the Sheep of Christ before they become believers ! Remember what Jesus said about those Sheep that would hear His voice in the future here Jn 10:16 16 And other sheep I have, which are not of this fold: them also I must bring, and they shall hear my voice; and there shall be one fold, and one shepherd. They were already His Sheep before they heard and believed, because remember Faith cometh by hearing, and when one believes they become the manifested Sheep of Christ or Sons of God Jn 1:12 ! A Couple of more examples, the Lost Sheep was still a Sheep of Christ, even while being Lost Lk 15:1-7, and likewise the Lost Son was still a Son while being Lost, thats why the Father said Lk 15:24 24 For this my son was dead, and is alive again; he was lost, and is found. And they began to be merry. That means the Lost Son was already in a Legal Union Relationship while dead and lost spiritually ! So again believers are sons of God before they become believers and that is why they shall become believers, because they must be confirmed into the image of Christ, their Firstborn Brother Rom 8:29 ! We know this also by Heb 2:17 17 Wherefore in all things it behoved him to be made like unto his brethren, that he might be a merciful and faithful high priest in things pertaining to God, to make reconciliation for the sins of the people. Christ in His humiliation/incarnation was made like unto His brethren, and this is brethren in the spiritual sense. Remember these brethren are also earlier called sons Heb 2:10 ! Now were all whom were His brethren believers when He first was born of the virgin mary ? The answer is no, the brethren of Heb 2:17 are the same brethren in Rom 8:29, and because He is the Son of God, His brethren are sons also, they are just unmanifested sons until regeneration, and because they are redeemed sons, the Spirit [and His fruit] are given them to manifest them as such Gal 4:6 6 And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father. By them also being Sons, they had to be redeemed Gal 4:4-5 4 But when the fulness of the time was come, God sent forth his Son, made of a woman, made under the law, 5 To redeem them[Sons] that were under the law, that we[Sons] might receive the adoption of sons. They receive the Spirit so that they also now begin doing the Will of their Father like their elder Brother Christ does Mk 3:35 33 And he answered them, saying, Who is my mother, or my brethren? 34 And he looked round about on them which sat about him, and said, Behold my mother and my brethren! 35 For whosoever shall do the will of God, the same is my brother, and my sister, and mother.