A. History
In general terms, the Tree of Life is good to eat. It makes man immortal. Man was forbidden by God to eat of the Tree of Knowledge, however. This prohibition was a test, which Man failed (according to the Catholic and Jewish understanding).
Original Sin was disobedience.
B. Symbolism
i. Eating of the Tree of Knowledge of Good and Evil was a violation of the Tree of Conscience, and God's prohibition against eating the forbidden fruit symbolises how even in a totally free and perfect world one must still be bound by conscience
There's a sense in which man's eating of the fruit of the Tree of Knowledge was a violation. Man's purpose was to tend the Tree of Knowledge, not denude it, and God exiled Man rather than allow him to violate in some way the Tree of Life. If the Tree of Knowledge was the Tree of Conscience then symbolically Man violated his conscience when he broke God's commandment not to do so: symbolically, man preferred to follow his ego rather than the voice of God within him. Tolkien makes use of the idea of the two trees being threatened or violated in 'The Silmarillion'. So does Neil Gaiman in Sandman.
ii. Knowledge = Gnosis, serpent = gnostic, Protestant, etc.
If the phrase 'of Good and Evil' is a merism then the Tree of Knowledge of Good and Evil was really 'the Tree of Knowledge, Good and Evil'. Actual knowledge, however, is true, justified belief. It is an appealing thought that the Trees of Knowledge and of Life could compositely be cognate with the Great Ash Tree in Norse mythology, which is a source of both life, in the form of the golden apples (which themselves have cognates in classical myth), and wisdom, which comes from the waters that well up underneath it. In Norse myth too, after all, the tree is threatened by a serpent (whereas in the classics the golden apples are guarded by one).
It is a similarly appealing thought that at the base of this idea was the use of hallucinogenic fruits by pagan magicians to gain shamanic wisdom and knowledge - the sort of thing that Yahweh would have roundly condemned. There may, indeed, be a parallel even with the Bodhi tree, which gives "wisdom", in Buddhism. The problem here is that the Tree of Knowledge was good to eat from. Indeed it is unthinkable that God would have allowed a tree that gave false knowledge to grow in Paradise. The Tree and the fruit itself therefore were not bad. But Man's purpose was to work in the Garden and to grow in sanctity through love, not merely through the attainment of knowledge. The forbidden fruit did indeed "open their eyes" then, but, according to Maimonides, what it really did was it altered their moral awareness, from one of truth and falsity to that of good and evil. And as Challoner points out, Man already knew Good and his temptation was to take an interest in wickedness. In the view of the gnostics, of course, and of the liberal Protestants such as Blake who are their intellectual successors, all knowledge is good, and therefore the Serpent itself was good, and Yahweh Himself is evil. Such are the Satanic roots of modern Protestantism.
iii. Tree of Knowledge = Carnal Knowledge, etc.
This is the least likely symbolic meaning, since procreation in the natural manner was not forbidden in Eden, as St Thomas demonstrates quite satisfactorily.
In the state of innocence there would have been generation of offspring for the multiplication of the human race; otherwise man's sin would have been very necessary, for such a great blessing to be its result. We must, therefore, observe that man, by his nature, is established, as it were, midway between corruptible and incorruptible creatures, his soul being naturally incorruptible, while his body is naturally corruptible. We must also observe that nature's purpose appears to be different as regards corruptible and incorruptible things. For that seems to be the direct purpose of nature, which is invariable and perpetual; while what is only for a time is seemingly not the chief purpose of nature, but as it were, subordinate to something else; otherwise, when it ceased to exist, nature's purpose would become void.
Therefore, since in things corruptible none is everlasting and permanent except the species, it follows that the chief purpose of nature is the good of the species; for the preservation of which natural generation is ordained. On the other hand, incorruptible substances survive, not only in the species, but also in the individual; wherefore even the individuals are included in the chief purpose of nature.
Hence it belongs to man to beget offspring, on the part of the naturally corruptible body. But on the part of the soul, which is incorruptible, it is fitting that the multitude of individuals should be the direct purpose of nature, or rather of the Author of nature, Who alone is the Creator of the human soul. Wherefore, to provide for the multiplication of the human race, He established the begetting of offspring even in the state of innocence.
Admittedly though it does set the theme for the rest of the Book of Genesis, which is primarily "about" generation. For the liberal Protestants, along with the idea of all knowledge being good, no matter how it is gained, just as appealing is the idea of sexuality being always good, no matter how it is exercised.[Summa Theologica, I.98.1]
