Adam would have been immortal had he obeyed God’s command, but he was like a small baby who does not know the difference between good and bad, and God would have played the role of the good father who takes care and chooses what is best for his child always. This is why the devil used this point in the discussion with Eve and Adam in order to make them fall in sin and contempt, and so get rid of this son who had priority in every respect. As seen in:

In this very same method you could deceive any small child or baby simply by saying to him, "It is not out of love that your father is telling you not to do this or that but because he is scared that he might lose his grip on you if you grow up and become experienced like him."

And after the fall in sin and contempt, Adam’s and Eve’s eyes were open, and God said:

Had Adam eaten of the fruit of the tree of life he would have become immortal, but he could do good or evil at any time, that is he could become an evil doing god. And since God is completely holy and does not do evil at all, so here the expulsion became necessary to make Adam and his descendants learn not only to know the difference between good and evil but also to choose good and despise evil at all times. And this could only be done by experiencing what evil can do to mother nature and everything in existence, and also the death of the body and soul which sin and contempt bring about.

Only after paying the full penalty of the sin in accordance with the requirements of the Godly justice could Adam and his descendents go back to heaven, if they choose good and learn to despise evil and stay away from it, and follow the path that God has laid out for them, that is to believe in and accept the redemption of the Son of God who takes their place in taking the just penalty of death they so rightly deserve.

To believe in the redemption of Jesus Christ is a totally essential condition that the believer may share in the eternal life of the Son of God, since man himself is but dead, and has no life in him, as seen in:

Now after that these three conditions are met, man can go back to heaven and stand once again in the presence of God and be there for ever, as Christ said:

6 .. 11 …, Holy Father, keep those you have given me in your name, that they may be one, even as we are one.
12 .. Sanctify them in the truth; your word is the truth.
13 .. 20 "I do not pray for these only, but also for those who believe in me through their word,
21 that they may be one; even as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me.
22 The glory which you have given me I have given to them, that they may be one even as we are one,
23 I in them and you in me, that they may become perfectly one, so the world may know that you have sent me and you have loved them even as you have loved me.

So you see, Christ disciples and those who believe in Christ through their words are to be made perfectly one in God himself, after they have been sealed with the seal of life, the seal of the living God upon their foreheads, and were sanctified with the holy pure blood which was spilled on the cross, and were born again of water and the Spirit and were given life through Christ’s blood and flesh. So Christ glorified be his name asks for their unity in the eternal Trinity of God, and to be perfectly one in it, so God will be all in all as in the beginning and before creating anything.

God created man and loved him so much that he united him in himself, so whatever difficulties met in this world are absolutely nothing compared to the actual glory awaiting the believers in Christ.