What is the meaning of the “gathered to his people” idiom?
Genesis 15:15 As for you [Abraham], you shall go to your fathers in
peace; you will be buried at a good old age.
Genesis 25:8 Abraham breathed his last and died in a ripe old age, an
old man and satisfied with life; and he was gathered to his people.
From the above verses arises the question of Abraham's "forefathers" or to whom "was Abraham gathered to his people .His ancestors were dead for thousands of years, and so his sons Isaac and Ishmael buried him with Sarah his wife.
Genesis 25:8-10 (NASB)
8 "Abraham breathed his last and died in a ripe old age, an old man
and satisfied with life; and he was gathered to his people. 9 Then his
sons Isaac and Ishmael buried him in the cave of Machpelah, in the
field of Ephron the son of Zohar the Hittite, facing Mamre, 10 the
field which Abraham purchased from the sons of Heth; there Abraham was
buried with Sarah his wife."
Abraham was gathered to his people, who were his people? His father was Terah and his brothers,Nahor and Haran from the Chaldean city of Ur. Abraham is an ancestor of Shem the son of Noah, Shem begat, Arpachshad - Shelah- Eber- Peleg - Reu- Serug and Nahor begat Terah the father of Abraham. (Genesis 11: 10-26) And so Abraham was gathered to all of his ancestors and buried at a good old age, therefore all of Abraham's ancestors are , in sheol/grave and are still awaiting the promise of resurrection of the dead that God promised his faithful servants.
King David also died and lied down with his forefathers: "Then David slept with his fathers and was buried in the city of David."( 1Kings 2:10 NASB) In a number of his Psalms King David spoke of deliverance from sheol/grave, read below.
Psalm 16:10 (NASB)
10 "For You will not abandon my soul to Sheol; Nor will You allow Your
Holy One to undergo decay."
Psalm 18:5 (NASB)
5 "The cords of Sheol surrounded me; The snares of death confronted
me."
Psalm 18:5 (NIV)
5 "The cords of the grave coiled around me; the snares of death confronted me."
Psalm 86:13 (NET Bible)
13 For you will extend your great loyal love to me, and will deliver my life from the depths of Sheol.
Acts 13:36 (NASB)
36 "For David, after he had served the purpose of God in his own
generation, fell asleep, and was laid among his fathers and underwent
decay."
On the day of the Pentecost the apostle Peter informed his listeners that David was still in sheol or grave, he said: “Brethren, I may confidently say to you regarding the patriarch David that he both died and was buried, and his tomb is with us to this day." (Acts 2:29) Therefore David did not ascend to the heavens and according to Peter’s words David’s resurrection is to be sometime in the future.
Note , Jesus also said: "No one has ascended into heaven, but He who descended from heaven: the Son of Man."John 3:13 (NASB)
The idiom “gathered to his people” means that, that they were all gathered in Sheol or Haʹdes, the common grave of mankind and from there, Revelation 20:13 tells us, the dead will resurrected.
Revelation 20:13 (NASB)
13 And the sea gave up the dead which were in it, and death and Hades gave up the dead which were in them; and they were judged, every one of them according to their deeds.