Genesis 2:17
NKJV - 17 "but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die."
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
"What did it mean to die?" I don't think it's been settled yet? The bible says, "The wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord" (Rom 6:23). That's what's known as a contrast: death and life.
Sin ends in death. It doesn't say 'not sinning will lead to living forever.' It says eternal life is obtained by it being given to the recipient. There's no other way!
Many in the church say death is not the cessation of life, it's just the separation of a soul from God for eternity. And life is then an eternity of torment. So that would make the wages of sin to be eternal torment, which is probably worse than to not ever have lived.
In other words, no one ever ceases to live; death isn't the end of life, it's the separation of the "dead" from the life of God, to be tortured and given an unending eternity to think about how big of a mistake it was to not receive the free gift of eternal life that was on the table.
Under that understanding, all life is eternal; It's either of bliss or torment. The "no longer alive" definition of losing life is meaningless if this is the correct meaning of death.
The "separation" meaning is plausible, but it doesn't jibe with the contrast between life and death.
Whoever has the Son has life; whoever does not have the Son does not have life (1 Jn 5:12). Does this describe death? We aren't born into the world with the Son. We arrive not having life.
There is a second death (Rev 20: 14).