You mkae an essential point. Ponder the following, if you will: God’s Spirit IS eternal life.. In Adam the Spirit of God was upon Adam and Eve, like a shawl, not in their spirit but upon their spirits. With the fall of mankind, the Spirit left the spirit presence in the first family. BUT GOD had a plan to put His Spirit IN the spirit of whomever of Adam’s descendants would ask for deliverance from the sin nature they inherited from Adam the progenitor. The human spirit that has His seed ABIDING in it (1 John 3:9) is alive forever more because it is God Who is in them, in their once dedness residing spirit.
Are you saying OT believers did not have the Spirit of God in them?