Of course the first person whose stature was demeaned by his misdeeds was Adam, who thus serves as the paradigm of the sort of a person who’d had a great potential that went unrealized . But to fully appreciate Adam’s perilous fall we’d first need to delve into the three phases of his life: the periods before his lapse and after it, and the future he could have had if he hadn’t lapsed .
Before he sinned, Adam was said to have been “crowned with such great and precious crowns of holiness that the angels above wanted to proclaim ‘Holy, Holy, Holy!’ in his presence” on the assumption that he was divine (see Breishit Rabbah 88:6). For being the very handiwork of G-d Himself, as Ramchal points out, Adam was not only brilliant, sagacious and holy, it was also true that “everything he did affected all of creation” since his presence was so momentous.
Yet he was destined to have been greater yet and even more exalted (if one could imagine) had he not rebelled against G-d’s wishes. In fact, had he not done that, then “everything that’s destined to happen after the Resurrection of the Dead” in the future “would have come about from the first, and without (our having to experience) death” which only exists because Adam lapsed, and could have been circumvented altogether, as Ramchal points out .
So, while he was greater at the point of creation than any of us are now, he demeaned himself and failed personally. Had he not sinned, Adam would have fulfilled his great and mighty potential and would have become “immortal and impervious to wrongdoing” as Ramchal words it. But he did indeed sin, and by doing so he not only failed to reach his potential, he also demeaned himself horribly and wound up being lower than he’d originally been .
Ramchal then makes the point that Adam’s state before he’d sinned was actually higher than his final state, yet lower than his potential state. And though he doesn’t say as much, it’s nonetheless true to say that Adam’s original state is actually comparable to our state before we’re born, when we’re all potential; his failed state is much like any one of our own failed states if we don’t reach our spiritual potential here in life; and Adam’s potential state is like our own will be when we — and all of humankind — will we’ll achieve true and full human potential.
Nonetheless, in His wisdom and so as to allow for free choice on all of our parts, G-d readied a state of being for Adam that would fit with his (and our own) imperfect state of being, which is reality as we now know it.
We’ll next explore the depths of Adam’s tragic downfall by going into his original state in even greater depth (which by extension will give us a glimpse into what might have been had he grown even greater, and which eventually be the state of all of humankind).
 While many people with great potential don’t fulfill it, Adam’s failing was especially tragic in that he established a pattern of failure for all of humankind when he could have stood on the highest grounds and brought about human, even cosmic perfection instead. As we’ll see, he was “this close” to success when he failed, which made for a greater failure than had he been much further away from it.
Ramchal will cite the example in the next chapter of two human beings who reached nearly as high as Adam could have and did not fail, Enoch and Elijah. Their high achievements were especially great given that they were far lower in stature than Adam to begin with and yet they managed to reach the heavens on their own.
See 3:12 and note 4 there for references to Adam.
 See Derech Hashem 1:3:8 where it’s pointed out that we, Adam’s descendants, have to do two things in the face of his plunge: not only do we have to achieve what he didn’t, but we also have to rectify what he’d ruined. See ¶152 below (at end) which addresses this point.
 That’s to say that Adam would have attained his place in the World to Come at once — along with all of humankind.
 And he thus became on par with us (though there are many of us who are higher than he was after the sin, and many who are lower).
Rabbi Yaakov Feldman has translated and commented upon “The Gates of Repentance”, “The Path of the Just”, and “The Duties of the Heart” (Jason Aronson Publishers). His works are available in bookstores and in various locations on the Web.