>called adopted sons

God (the Husband) was required to kill his adulterous Bride, Israel (and therefore all Israelites), under the law (the Vow between the two).
This is why Christ speaks about adultery right after talking about the law being fulfilled. It's the same subject, yet to say everyone will be saved *and say* the law would be fulfilled sounded ludicrous. (The law required all of Israel be killed! Yet all Israelites would be saved? It could not be reconciled... unless...) God came as a man and died.
When the Husband dies, his Bride is released from her vow. (This is why Paul talks about an unfaithful woman being released from the law by the death of the husband (Romans 7) in the middle of his speech on Christ and the law.) "Till death do us part."

The same prophets in scripture used by God to formally accuse Israel of adultery also formally stripped the Israelites of being his people, now to be called "not my people" and "no mercy", with God asking where "their mother" (his Bride) had any paper of divorce after her unfaithfulness.
He also called these "not my people" his sheep scattered to every mountain--but that he would keep a remnant (those that were in Judea by the days of Christ) and that he would be their shepherd himself. (Christ being the Good Shepherd is reference to what he had said through these prophets.)
Yet through the very same prophets, God promised the Israelites that they would once more be called "my people" and those named "no mercy" would have mercy; that they would once more be adopted as sons; that he would bring a new covenant (which seemed wild because it's against the law for a Husband to marry a Bride he has divorced). It really isn't a wonder why people ridiculed these prophets, if you consider the seemingly contradictory things they said.

So by the death of the Husband, the Bride was spared. Her sins--all of their sins--were forgiven. The Husband had taken these on--in that he who did not deserve to die (he who had been faithful) did die. (This is also one part of the reason it was important that Christ not have a father by the flesh. He was born of a woman (as promised to Eve), of Israel, of the line of David, but not, according to the law, under a father--thereby born of Israelites (as promised) but not guilty. He was then also free to be a kinsmen redeemer.)

The Israelites (the sons) were adopted as sons once more by the new covenant. With the resurrection, God could (not *remarry*, as he had suffered death, but instead lawfully) marry the Bride. This new covenant will come in full at the Wedding Feast, the Last Day. That covenant will be everlasting, and we will be sons.

Notice:
Since the Bride was released, ALL Israelites (all of the sheep scattered upon every mountain as lamented through the prophets, all of the sheep "not of *this* fold (Israelites knowing they were Israelites, trying to keep the law, Judeans and Samaritans)"--meaning sheep (Israelites) scattered through nearly a millennium by the Assyrian deportation and the Babylonian and the Roman--these Israelites who wouldn't even know they were Israelites but were still under the law were also released). The wild olive branches (olive tree being a repeated symbol of Israel throughout scripture) could be grafted back into the domesticated olive tree with the new covenant.

What's more:
Those scattered Israelites had fled or been deported throughout the nations, for centuries upon centuries, as seen by the Old Testament (and secular historians as well).
Those Israelite fathers had children who had children who had children... for centuries.
In other words, the seed of Abraham became many nations.

And Christ was to be a light for the revelation of the nations, as the priest Simeon prophesied immediately upon seeing the Christ child. He was to reveal the nations he'd promised to Abraham. The Great Commission is Christ sending out those eleven disciples to do so.

That is the adoption as sons.

It's the Prodigal Son story.

A note:
(The Greek of what Simeon says of Christ is literally "a light for the revelation of the nations" as I write above. (All I can do is point to the explicit Greek. It's as clear and different in Greek as it would be in English. It's wild.) It is interesting that it was ever translated instead as "a light *of* revelation *to* *the Gentiles*", while not surprising that it continuous to be accepted "because it's written" and has been passed down (only since) as the same error, much like "born again" is passed down when the verse says explicitly "born of above."
Translation changes that change meaning blatantly--an example being "dog" to "non-Huskie"--or "nations" to "Gentiles" (a new, made up word meaning "non-Judeans")--are the same successful twist as making money based on nothing and removing women from their femininity. Not everyone/thing twisting is a liar. Most are just twist*ed*.)