In the Bible, there are only a handful of people and two groups that are called God's Sons. The first group were the faithful angels. "Where did you happen to be when I founded the earth? Tell me, if you do know understanding...When the morning stars joyfully cried out together, and all the sons of God began shouting in applause?" (Job 38:4, 7) Further, Israel as a group is called a Son of God.-Deuteronomy 14:1; Isaiah 43:6
The people that are called Sons of God are Adam, Solomon, and Jesus. (Luke 3:38; John 10:36) Solomon is a special case. He was called God's son when he was king of Jehovah's nation, Israel. And even then, he was not called God's son until God bestowed that title upon him, showing that this was not something he was from the very beginning of his life. (2 Samuel 7:14) The nation of Israel was a collective son of God because they were a "holy people...special property, out of all peoples" and because Jehovah had helped them along from the very beginning after He made the covenant with Abraham.-Deuteronomy 14:1-2*
After Israel lost this privilege, it was given to others--Anointed Christians. They were not, however, full sons of God. "God sent forth his Son, who came to be out of a woman and who came to be under law, that he might release by purchase those under law, that we, in turn, might receive the adoption as sons." (Galatians 4:4-5) Did all humans become adopted sons of God after Jesus' death, as some may argue the Scripture as saying? Paul says, "You are all, in fact, sons of God through your faith in Christ Jesus."-Galatians 3:26; 4:6-7; John 1:11-12
Now, we must find out if this applies to all Christians, Anointed and the Great Crowd, or only to one group. Do we all become God's sons (by adoption) upon faith in Christ? No.
"For all who are led by God’s spirit, these are God’s sons. For you did not receive a spirit of slavery causing fear again, but you received a spirit of adoption as sons, by which spirit we cry out: 'Abba, Father!' The spirit itself bears witness with our spirit that we are God’s children. If, then, we are children, we are also heirs: heirs indeed of God, but joint heirs with Christ, provided we suffer together that we may also be glorified together." -Romans 8:14-17Paul promises that these will be joint heirs with Christ and that they will be glorified, both of which do not apply to the Great Crowd.
Now, these Anointed Ones are not full sons of God, but only adopted sons. Will they be in this state forever? No. "[God] has also put his seal upon us and has given us the token of what is to come, that is, the spirit, in our hearts." (2 Corinthians 1:22) One day they will be full sons of God while now patiently waiting and gladly being sons of God by adoption.
However, there is more! John says that Anointed Christians are "Firstfruits". This means that there must be ones who receive blessings afterwards. Yes, one day, all humans who pass the final test mentioned at Revelation 20:7-10 will be able to become earthly sons of God! Romans 8:19-23 is an indication that those with the earthly hope will one day become children of God.
One other Scripture that proves that point is found in the book of Revelation.
"With that I heard a loud voice from the throne say: 'Look! The tent of God is with mankind, and he will reside with them, and they will be his peoples. And God himself will be with them. And he will wipe out every tear from their eyes, and death will be no more, neither will mourning nor outcry nor pain be anymore. The former things have passed away...anyone conquering will inherit these things, and I shall be his God and he will be my son.'" -Revelation 21:3-7The context shows that those who reside on earth will be God's son.
So, while we are not sons of God as many people claim due to our being separate from God due to sin, this will not be the case forever. In fact, one day we, whether or the Anointed or the Great Crowd, will all have the "glorious freedom of the children of God!"-Romans 8:21
*The Israelites lost their position as a collective "son" after they lost his favor. (See Matthew 23:37-38; John 8:41-47) This is indication that sonship was not just from being descendants of Adam or Abraham but on their spiritual relationship.
 Insight on the Scriptures, Vol. 2, p. 999, pp. 5-7