In Genesis 2:15-17 we read:
And the LORD God took the man, and put him into the garden of Eden to dress it and to keep it. And the LORD God commanded the man, saying, Of every tree of the garden thou mayest freely eat: 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.
To eat from the tree of knowledge of good and evil means to know - to decide for yourself - what is good and what is evil. To "eat" from that tree is to usurp God's authority, to take God's place as the decider of good and evil. God alone is perfect and righteous, so God alone is qualified to determine right from wrong, good from evil. When Adam and Eve decided to "know" or determine good and evil, they were presumptuous: they presumed they could do what God alone could do. In essence - in this respect - they presumed to take God's place.
And the serpent said unto the woman, Ye shall not surely die: For God doth know that in the day ye eat thereof, then your eyes shall be opened, and ye shall be as gods, knowing good and evil. (Genesis 3:4,5)
The couple tried to hide from God. And what did God do?
And the LORD God said,
Behold, the man is become as one of us, to know good and evil: and now, lest
he put forth his hand, and take also of the tree of life, and eat, and live
for ever:
Therefore the LORD God sent him forth from the
garden of Eden, to till the ground from whence he was taken. (Genesis
3:22,23)
So Adam and Eve were put out of the Garden, without access to the tree of life. They no longer had any hope of living forever. They were going to die, as God had warned them.
Did they have immortal souls? No, the Bible says nothing about having an immortal soul; that's actually a pagan concept not found anywhere in the Bible. There are some who reason that we were made in the image of God, and because God is immortal, they conclude we must be immortal as well. But that is not what we see in Genesis. Adam and Eve would never have needed access to the tree of life if they were already immortal!
Perhaps you noticed the "us" in the quote above, implying God is plural. The same "us" appears in Genesis 1:26: ". . . let us make man in our image." Let me explain as briefly as possible. The "us" is the Father and the Word; the Word, through whom creation was done, later became flesh - the man Jesus - as related in John 1:1-3 and 14:
In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God. All things were made by him; and without him was not any thing made that was made . . . And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father, full of grace and truth.
In a prayer Jesus confirmed His existence with the Father before creation:
And now, O Father, glorify thou me with thine own self with the glory which I had with thee before the world was. (John 17:5)
Jesus and the Father are of one mind, have one will and one plan. Jesus said:
I and [my] Father are one. (John 10:30)
Now, regarding Satan's activity in the Garden: by violating God's instruction on what was right and wrong, Satan himself was presuming to know right from wrong. Satan was the leader in the revolt against God. Isaiah gives more insight into this, showing that Satan, apparently a long time in the past, had actually presumed to take God's place:
How art thou fallen from heaven, O Lucifer, son of the morning! how art thou cut down to the ground, which didst weaken the nations! For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north: I will ascend above the heights of the clouds; I will be like the most High. Yet thou shalt be brought down to hell, to the sides of the pit. (Isaiah 14:12-15)
God has temporarily set us on our own, under Satan's influence, to let us experience the consequences of our own decisions. We are now free to exercise whatever political philosophies, government systems, social and economic structures, utopian schemes, religious beliefs and so forth that we believe will work best for us. God knew that there would be no other fair, honest and convincing way to prove what was true and right. As events run their course we will ultimately bring ourselves to the brink of self-destruction in what the Bible calls the "great tribulation" during "the time of the end." Then God will intervene and save us from self-destruction. For the next thousand years Satan will be put aside (Revelation 20:1-3) and God will show all mankind His ways as the Kingdom of God rules right here on earth:
And in the days of these
kings shall the God of heaven set up a kingdom, which shall never be
destroyed: and the kingdom shall not be left to other people, but it shall
break in pieces and consume all these kingdoms, and it shall stand for ever.
(Daniel 2:44)
And it shall come to pass in the last days, that
the mountain of the LORD'S house shall be established in the top of the
mountains, and shall be exalted above the hills; and all nations shall flow
unto it. And many people shall go and say, Come ye, and let us go up to the
mountain of the LORD, to the house of the God of Jacob; and he will teach us
of his ways, and we will walk in his paths: for out of Zion shall go forth
the law, and the word of the LORD from Jerusalem. And he shall judge among
the nations, and shall rebuke many people: and they shall beat their swords
into plowshares, and their spears into pruninghooks: nation shall not lift
up sword against nation, neither shall they learn war any more.
(Isaiah 2:2-4)
Even while allowing us to experience the consequences of rejecting His instructions, God is exercising a plan of redemption and salvation. Ultimately, all who choose God's ways will be able to eat from "the tree of life"; they will enjoy eternal life with God as His sons and daughters, His family. In the Revelation this promise was confirmed:
He that hath an ear, let him hear what the Spirit saith unto the churches; To him that overcometh will I give to eat of the tree of life, which is in the midst of the paradise of God. (Jesus, in Revelation 2:7)