The answer for that lies back in the book of beginnings. Consider the scripture:
15 Then the Lord God took the man and put him in the garden of Eden to tend and keep it. 16 And the Lord God commanded the man, saying, “Of every tree of the garden you may freely eat; 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.”
God had decreed that man could eat of anything in the garden – but man could not eat of the tree of knowledge of good and evil. God had decreed that should man do this, he would surely die. Now, let’s answer a very powerful question – why was the tree of knowledge of good and evil so important? Did God just pick some random tree and plant it in the middle of the garden to trip up man? Why was God SO CONCERNED about the tree of the knowledge of good and evil – and not, for example the tree of life (which was ALSO there – see Genesis 3:22)? The answer lies in a totally different set of scripture – specifically Romans 7:7-12:
7 What shall we say then? Is the law sin? Certainly not! On the contrary, I would not have known sin except through the law. For I would not have known covetousness unless the law had said, “You shall not covet.”8 But sin, taking opportunity by the commandment, produced in me all manner of evil desire. For apart from the law sin was dead. 9 I was alive once without the law, but when the commandment came, sin revived and I died. 10 And the commandment, which was to bring life, I found to bring death.11 For sin, taking occasion by the commandment, deceived me, and by it killed me.12 Therefore the law is holy, and the commandment holy and just and good.
Basically, in this scripture, Paul is speaking of the relationship between KNOWLEDGE and SIN. In the example that Paul is using, he explicitly states that sin came because he KNEW that to covet was a sin. If you look at the boldface passage, you see that when he (Paul) heard the commandment against covetousness, then, as he said “sin revived and I died”. Basically, without the knowledge of covetousness, there could be no sin.
Back in the garden, the scripture explicitly says “And they were both naked, the man and his wife, and were not ashamed.” (Genesis 2:25). But look what happens right after they eat of the tree of KNOWLEDGE of good and evil:
10 So he said, “I heard Your voice in the garden, and I was afraid because I was naked; and I hid myself.”
11 And He said, “Who told you that you were naked? Have you eaten from the tree of which I commanded you that you should not eat?”
In this passage of scripture – by eating of the tree of knowledge of good and evil, man became aware of sin (in this case, he realized that he was naked) – and, as Paul clearly states “sin revived, and I died”. The reason that eating the tree was forbidden was simple – by eating of it, man would become aware of good and evil – and would therefore become aware, and thus, would become subject to sin.
You see, the serpent (Satan) didn’t just tempt man with disobedience – he opened the door for man to understand that there was such a thing as EVIL – and thus, he opened man’s eyes to sin.
Because of this, man was now aware – and thus, he died. Now, you may ask, “But wait a minute – Adam didn’t die – he lived for hundreds of more years”. Yes, but his relationship – his close interaction - with God had died. Look back at Genesis 3:10 – when God came looking to continue His relationship with man – man ran away. Man was now fully aware of sin – the tree had done that – and now he knew that relationship with God like before was impossible. Adam knew that he had sinned – not when God told him – he knew it before.
We see here an example of why sin was so damaging – it separated man from God. As a matter of fact, we know that man – no matter how righteous he might try to be, could never have total relationship with God. Consider the scripture:
15 Then he said to Him, “If Your Presence does not go with us, do not bring us up from here. 16 For how then will it be known that Your people and I have found grace in Your sight, except You go with us? So we shall be separate, Your people and I, from all the people who are upon the face of the earth.”
17 So the Lord said to Moses, “I will also do this thing that you have spoken; for you have found grace in My sight, and I know you by name.”
Here we see Moses making a request to God – he was asking that God go with the children of Israel as they made their way through the wilderness. God directly says “I will do this thing” – and He says that He would do it “because you have found grace in My sight”. So, Moses had an absolutely incredible relationship with God – and yet consider the rest of this scripture:
18 And he said, “Please, show me Your glory.”
19 Then He said, “I will make all My goodness pass before you, and I will proclaim the name of the Lord before you. I will be gracious to whom I will be gracious, and I will have compassion on whom I will have compassion.” 20 But He said, “You cannot see My face; for no man shall see Me, and live.” 21 And the Lord said, “Here is a place by Me, and you shall stand on the rock. 22 So it shall be, while My glory passes by, that I will put you in the cleft of the rock, and will cover you with My hand while I pass by. 23 Then I will take away My hand, and you shall see My back; but My face shall not be seen.”
God was willing to commit Himself to going with the children of Israel (who He had just, incidentally, planned to destroy because they were disobedient) at the request of Moses – but God COULD NOT let Moses see His face. Not WOULD NOT – but COULD NOT. You see, God could not let Moses see His face – because it would destroy him. God would have been perfectly willing to do it – but it would have destroyed Moses.