From Sonship of Israel to Believers

Sonship in the Bible is an office before it is a feeling.
Adam is God's son. Israel is God's son - Pharaoh is told so directly. The king of Israel is called God's son in Psalm 2. Each time, being a son means holding a position: representing God, ruling on his behalf, belonging to his household.
One clarification worth making. Israel is called firstborn, but God's firstborn human creation is actually Adam. Firstborn here is rank, not sequence.
Then the New Testament takes the same language and hands it to believers - adoption, children, heirs. Paul says the real children of Abraham are those who belong to Christ, whether Jew or Gentile.
Scripture links:
- Exodus 4:22 - Israel is my firstborn son.
- Psalm 2:7 - you are my son, today I have begotten you.
- Galatians 3:26-29 - in Christ you are all sons of God through faith, and heirs according to promise.
You were not adopted into a mood. You were adopted into an office.