Genesis 2:17 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.
Adam's use of the garden of Eden was conditional. Like any conditional covenant, there was a clause detailing what will happen if the conditions are not met. In this case the penalty for breaking the covenant was immediate execution.
Some may say this is a difficult verse because, they observe correctly, Adam did actually break God's law, but the penalty of death was not executed as stated. To answer this glaring problem we might assume that “day” in verse 17 means a long period of time because Adam did die later (roughly 339,449 days later). But there are those who will argue that the word day used in the previous chapter for the seven day creation is a 24 hour period. They would be right and it is the same with this usage. Adam, using the language definitions God placed in His mind, would have believed he would die within the 24 hour period surrounding his disobedience. That is the common biblical usage of the word day, especially in the book of Genesis.
Perhaps we could hypothesize that Adam only died spiritually in that 24 hour period. Hmmm – Putting “spiritual death” (and any combination of those words or related words) into my Bible program search function finds no place in the Bible where it says that Adam exclusively died spiritually on that day. A. W. Pink said, “It is not said, “thou shalt die physically,” nor “thou shalt die spiritually,” but simply thou shalt surely die. The absence of any modifying adverb shows that the term death is here taken in its widest scope, and is to be defined according to whatever scripture elsewhere signifies by that term. It is the very height of presumption to limit what God has not limited.” The verb translated “die” appears in 64 verses in Genesis and every use is exactly as Pink said.
So the real question becomes: why did God not do what He said He would do? If we look carefully throughout the Bible we find this same cycle repeated over and again. God's law with its penalties is communicated clearly to someone by various means. They break the law. God declares their guilt. But the penalty required by justice is not immediately executed. Sometimes, we are told, people are permanently freed from their guilt and its penalty. How does that work?
The reason justice was stayed is Gospel mercy entered the scenario. God didn't do what justice demanded for Adam because another person committed to fulfill the demands of justice on behalf of Adam. Another person, Jesus the incarnate Son Of God, would by His death, burial, and resurrection secure forgiveness and grace for Adam. So Adam lived to see another day because Jesus would secure mercy on his behalf.
The reason Adam and Eve didn't die the very day they disobeyed God is because forgiveness was introduced into the world on that same day. God forgave Adam and Eve based on an everlasting Covenant which included Jesus Christ's gospel work on behalf of sinners. The reason we, Adam and Eve's children, are allowed to live is because this mercy of God in Jesus Christ still rules the earth, so justice is withheld by mercy. But this common grace, by which all sinners live on God's earth, gives way to eternal justice and death unless personal faith has laid claim to the Gospel of Jesus Christ.