Genesis 2:17

17 But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.

Genesis 5:5

5 And all the days that Adam lived were nine hundred and thirty years: and he died.

God says that Adam will die the day he eats from the tree, yet he eats from the tree (Genesis 3:6), and lives to be 930 years old (Genesis 5:5).

The answer is found in understanding that the Bible teaches that when a person sins, their spirit dies (Romans 6:23, 7:9). That is why, for each person, the new birth is required (John 3:3-7). When a person is born again, they are described as being raised from the dead, and passed from death unto life:

Ephesians 2:1

1 And you hath he quickened, who were dead in trespasses and sins;

John 5:24

24 Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.

Therefore, I believe that Genesis 2:17 is the first instance of the Biblical doctrine of spiritual death, in distinction from physical death.

It is also worth noting that the skeptic's attack on Genesis 2:17 is quite silly. The author of Genesis, in chapters 3 and 4, recorded Adam and Eve eating from the tree, and then going on to live. Are we to believe that he immediately forgot Genesis 2:17? Or is it more likely that the critic is ignoring genre, and dealing unfairly with a text which they have no intention of believing?