When we first read a text in the Bible it seems our natural inclination is to rake it at face value. However the context of a text is important as well as some other background information. Bellow is some insight into the relationships of first born sons and background information that might lead to a better understanding as to why called others his sons.
But first Adam could be called a son of God because he was the “crowning achievement“ of Gods earthly creation.
Jesus is known as the Son of God because he was gods son here on earth as well of the firstborn of ALL creation.
Now according to God’s promise to Abraham (the Abrahamic covenant), His due time had arrived for Him to deliver the nation of Israel from “the iron furnace” of Egypt. God considered Israel as his firstborn son by virtue of the promise to Abraham. When Jacob went down to Egypt with his household, he went down voluntarily but his descendants later became slaves. As a nation, they were dear to God as a firstborn son, and God had the legal right to deliver them from Egypt without the payment of a price. (De 4:20; 14:1, 2; Ex 4:22; 19:5, 6.)
At Romans 9:4 Paul speaks of the fleshly Israelites as those “to whom belong the adoption as sons and the glory and the covenants and the giving of the Law,” and this evidently refers to the unique position granted Israel while they were God’s covenant people. Thus, God, on occasion, spoke of Israel as “my son.”
From earliest times the firstborn son held an honored position in the family and was the one who succeeded to the headship of the household. He inherited a double portion of the father’s property. (De 21:17) Reuben was seated by Joseph at a meal according to his right as firstborn. (Ge 43:33) But the Bible does not always honor the firstborn by listing sons according to birth. The first place is often given to the most prominent or faithful of the sons rather than to the firstborn.(Ge 6:10; 1Ch 1:28; compare Ge 11:26, 32; 12:4; )
The firstborn came into considerable prominence at the time that God delivered his people from slavery in Egypt. Among the Egyptians, the firstborn were dedicated as sacred to the sun-god Amon-Ra, the supposed preserver of all the firstborn. The tenth plague that God brought upon the Egyptians served to discredit this god and showed up his inability to protect the firstborn. By obeying God’s instructions concerning the slaying of a lamb and the splashing of its blood on the doorposts and upper part of the doorway of their houses, the Israelites did not lose their firstborn in death, whereas all the firstborn of the Egyptians, of both man and beast, were slain. (Ex 12:21-23, 28, 29)
Since the firstborn sons among the Israelites were those in line to become the heads of the various households, they represented the entire nation. God, in fact, referred to the whole nation as his “firstborn,” it being his firstborn nation because of the Abrahamic covenant. (Ex 4:22) In view of his having preserved their lives, God commanded that “every male firstborn that opens each womb among the sons of Israel, among men and beasts,” be sanctified to him.
God also designated certain individuals within Israel as his ‘sons,’ in a special sense. Psalm 2, attributed to David at Acts 4:24-26, evidently applies to him initially when speaking of God’s “son.” (Ps 2:1, 2, 7-12) The psalm was later fulfilled in Christ Jesus, as the context in Acts shows. Since the context in the psalm shows that God is speaking, not to a baby, but to a grown man, in saying, “You are my son; I, today, I have become your father,” it follows that David’s entry into such sonship resulted from God’s special selection of him for the kingship and from God’s fatherly dealings with him. (Compare Ps 89:3, 19-27.) In a similar way God said of David’s son Solomon, “I myself shall become his father, and he himself will become my son.”—2Sa 7:12-14; 1Ch 22:10; 28:6.
Leah was the first one to marry and bear children for Jacob, but it was so only by a deception worked on Jacob. Rachel was the one he loved and bargained for, and hence it was to Rachel’s offspring that the birthright was to go, though Jacob’s offspring by Leah were older. (Gen. 29:18-28) Sarah was the beloved wife of Abraham, and it was to her offspring Isaac that the birthright went, even though Abraham’s son Ishmael by Hagar was older. So it was with Rachel’s offspring Joseph. However, Joseph became no tribal head in Israel, but his sons Manasseh and Ephraim did. Manasseh was the older, but divine direction caused the better blessing to go to Ephraim. Of him God said: “Ephraim is my firstborn.” (Gen. 48:8-20; Jer. 31:9) However, the tribe of Ephraim later eliminated itself from this favored position by many derelictions, and the psalmist tells of God’s action: “He refused the tabernacle of Joseph, and chose not the tribe of Ephraim: but chose the tribe of Judah.”—Ps. 78:9, 67, 68.
With Joseph, by Ephraim’s failure, eliminated, Rachel’s other offspring, Benjamin, had to have its chance. That opportunity came with Saul’s anointing as king, for Saul was a Benjamite. At 1 Samuel 13:13 it speaks of the possibility of Saul’s kingdom being established forever; but we must remember that the Hebrew word here translated “forever” is ohlahm. As shown in previous Watchtowers and in the “Let God Be True” book, this Hebrew word means a period of concealed or indefinite time, not necessarily for everlasting. True, Jehovah God knew before time that the kingdom would not remain in the house of Benjamin; but it was Saul’s own presumptuous and faithless course that caused his loss of the kingship for his household and tribe. The mere exercise of Jehovah’s power of foreknowledge did not actively force Saul to act reprehensibly. On his own Saul acted contrary to the express commands of Jehovah God, fully responsible for these violations in the face of knowledge of his sins.
With the favored Rachel’s offspring having had their chance, the older sons of Leah would be in line for the blessing of kingship. Ahead of Judah were Reuben, Simeon and Levi. All three of these were eliminated by the occurrences mentioned by Jacob at the time of his blessing of his sons. (Gen. 49:3-7) Moreover, later the Levites acted in noteworthy faithfulness and were rewarded by being given the blessings of the priesthood. That would prevent any of their number from becoming king. So Judah was now next in line, and the prophecy at Genesis 49:8-12 shows he would succeed in gaining the kingship, and being the human ancestor of the King who will reign forever, Christ Jesus. Of course, in all this Jehovah was under no obligation to conform to the general practice concerning the firstborn and birthright privileges. He could have chosen whomever he wanted, right at the outset, without eliminating those first in line according to human procedures.
