There are various aspects to death and various ways it is pictured in scripture; we see both physical death and spiritual death.
But the first mention of death in the bible is not as clear as some might think.
Compare versions:
Genesis 2:16–17 (ESV)
And the LORD God commanded the man, saying, “You may surely eat of every tree of the garden, 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.”
Two phrases stand out when asking the question asked in the Opening Post.
1) "in that day" – Does "day" mean 24 hours? Or is the phrase an idiom meaning "from that very instant" or "from that time"?
2) "surely die" is often overlooked and misinterpreted. The Hebrew text uses the word "die" twice, some literally translate the Hebrew as "dying, you shall die"
Here are two modern translations often used for their stricter adherence to the structural form of the original language. Both authors are Jewish scholars.
Genesis 2:16–17 (The Five Books of Moses – Everett Fox, 1984, 1995)
YHWH, God, commanded concerning the human, saying: From every (other) tree of the garden you may eat, yes, eat, but from the Tree of the Knowing of Good and Evil— you are not to eat from it, for on the day that you eat from it, you must die, yes, die.
Genesis 2:16–17 (The Five Books of Moses – Robert Alter, 1996, 2004)
And the LORD God commanded the human, saying, "From every fruit of the garden you may surely eat. But from the tree of knowledge, good and evil, you shall not eat, for on the day you eat from it, you are doomed to die."
The understanding is that sin introduced the process of death from the moment it happened.
Paul writes that "sin entered the world through one man, and death through sin" (Romans 5:12), and "the wages of sin is death" (Romans 6:23).
Is Paul speaking about spiritual or physical death? …or is it less clear?
I believe we see the "spiritual" interpretation of death prominently articulated in Augustinian theology. This is when the idea of "spiritual" death overtook the idea of "physical" death and eclipsed it in many theologians teachings.
Rob