This is a question I've heard a number of times. Generally it comes up because many of the Church Fathers used language similar to what Neo-Platonic thought used. The assumption is often made without looking into exactly how the Church Fathers used that language, or comparing it with actual Neo-platonic writings or looking at the significant ways it was modified.

It's worth noticing how in his epistles St Paul uses the Platonic formula of body, spirit, and a soul comprised of mind, will, emotions. Similarly, St John's Gospel co-opts first-century gnostic language to communicate Christianity, as the book Christ the Eternal Tao does with ancient Eastern religious language. 

An interesting note from my e-mail:

...Nor do they consider the idea that people of that time simply had a certain world view in which these concepts made sense, and thus there was need to speak and use them. For instance, it is highly possible that within another 500-1500 years or so (things are changing so fast now days that this could even be shorter), assuming the Lord tarries, people will look back at the words of many of today's most popular Protestant, and even Orthodox writers and say that we were highly influenced by New-Age and secular humanism and Kantian philosophy... simply because those things, whether we recognize it or not, have created a certain language and world view that we think about reality within.

Whether or not a certain individual really has allowed one of the philosophical systems (or any other number of them) to be more than a vehicle for communication to the current world and conformed the faith to a man-made system or not, it will appear on the surface that we have simply because we use much of the same language and thought patterns of these philosophies in our thinking about things. In some cases I'm sure it is true, in others it may just be someone using the language, but using it in a Christian understanding and context.

To this point, I would say that there are more elements in much of Protestantism that can be attributed to Neo-platonic thought than there are in Orthodoxy. Partly because a key understanding of what the fall was about within the Western world was so influenced by Augustine, and the way he came to the conclusion about that was based more upon his philosophy than revealed doctrine.

Were Adam and Eve in a state of absolute perfection at their creation? I think much of classical Western theology has thought so (correct me if I'm wrong here, but I have not personally run across any wide streams of theological thought to the contrary on this point though I'm sure there are some out there somewhere) based the concept of the fall and original sin on these points, that the understanding of this owes more to Neo-Platonism than Biblical revelation.

For instance, where in the Bible does it say that Adam and Eve were created in a state of absolute perfection? Is this not a deduction from the idea that God is perfect and could only create something that is likewise perfect? In some cases, I'm sure that would be the argument. However, when Augustine was trying to discern what the state of Adam and Eve were like before the fall, he based his knowledge of this upon a deduction that there state was an exact opposite of what ours is. This is due to the concept of emanations from the "One", or in this case God (Augustine did adapt that to Christian thought) and that Adam and Eve were created in this state of perfection right around God. At the fall, we fell down several emanation levels to a more material state, a state with more darkness, sin (the absence of God) and consequently to less perfection. It was a corruption of the previous state of perfection, dominated by spiritual existence right next to God.

Some of the things Augustine deduced from this view are no doubt true. However, the Fathers are clear that Adam and Eve were born in a state of immaturity, and were to grow further. Their next step was to grow enough that they could partake of the Tree of the Knowledge of Good and Evil, and once there would be able to partake of the Tree of Life. (St. Ephraim the Syrian) Maybe Augustine didn't have access to them, as I understood he did not know Greek and certainly not Syrian, and I guess there were not much in the way of Latin translations available... I don't know. However, using deductive logic based upon a certain philosophical system, he came up with an understanding which differed significantly from that of what had been understood on the subject. The interesting thing is that at least in much of Protestant theology this seems to be the standard view of their state before the fall, despite the fact that it cannot be found in the Bible.

How this affects what original sin is especially in Augustinian thought, shows in what manner this one concept has affected much of Western theology in this regard. Original sin, being the existence down several "emanations" from God, is the living in our current material, changing and imperfect existence. Because of this "imperfection", we have this tendency to sin and we cannot escape from it ourselves, and we are thus "guilty" of it. There is more of a dualism of spirit and physical (from which the rejection of the 7th council finds its seeds). While the material was not "bad", it was, however, the absence of God and spiritual life, and thus was opposed to it. Movement back to God was to become less material, less imperfect and greater degrees of sinlessness. The material existence was seen as a corruption and that which passed on this "original sin" which was the source of our tendency to sin, making it hereditary.

Thus original sin became a "substance" in the sense of something in us that needed eradicating or cleansing, in as much as we had a need to move from that to the more perfect spiritual state filled with God, and thus no "original sin".

Of course, any one Protestant or group will not necessarily adopt all this, but it seems the idea of Adam and Eve being in a state of perfection before the fall is predominant, and must affect original sin and our salvation from it. To some degree I'm exploring those links. However, that is why I say that Western theology has tended to be based more upon a Neo-platonic derived belief concerning a key doctrine than has the East, even if the East can be seen as using the language more.

That aside, it is hard for man to go beyond how he thinks and views the world, and we have the irresistible tendency to want to go beyond just understanding, and fit God and who we think He is within our understanding. Much of this is done without thinking overtly about it, at least for most people, because that is where real philosophy operates. Not so much in the overt discussion and intentional use of it, it is simply how we think and view "reality". Thus, we tend to attempt to fit all reality, even that concerning God, into it. It is only when we understand that this is exactly what we are accusing the Fathers of doing, that we realize we have to adopt the acknowledgement of speaking about God knowing the limitations of our language and philosophy and allow for "mystery", not as a cop out when we are lazy or want to avoid unpleasant realities which deny our point of view, but in simply acknowledging that God's ways and view of reality (which is the only "correct" view of reality) are at odds with our view of it, which is limited and finite like we are. There are points of contact, I'm sure, but without seeing things the way He does, we cannot ever hope to deduce the truth of His reality, which is why we rely on revelation so much. Seek to understand and communicate it, but not to create it or "develop" it...