Reb Yonason Eibshitz zt"l explains that Hashem's acceptance of teshuva is analogous to techias ha'meisim - resurrection of the dead. This is because the Torah refers to a wicked person, even during his lifetime, as if he were dead. Only once he repents, and Hashem accepts his teshuva, can he be considered alive, and if so he has in effect been resurrected.
Rav Wolbe (Ma'amarei Yemei Ratzon pg. 340) writes that with this in mind we can understand an amazing statement of Chazal regarding the immense power of teshuva. Chazal (Yalkut Yechezkel 18) tell us that a question was posed, "What could be done [to rectify] the soul that has sinned?" "Wisdom responded, 'Evil will pursue the sinner.' They asked prophesy and she answered, 'The sinner must die.' They asked the Torah and she replied, 'He should offer a sacrifice and he will be forgiven.' They asked Hashem and He said, 'He should do teshuva and he will be forgiven.'"
Wisdom understood that one aveirah leads to another aveirah and, therefore, a transgressor creates a whirlpool of sin, and he will forever be pursued by sin. Prophesy recognizes the awesomeness of The Creator and rightfully feels that anyone who has rebelled against Him has forfeited his right to live. In contrast, the Torah acknowledges the ability to rectify the sin. Nevertheless, it seems that the rectification is limited to a sin performed unintentionally since only such a sin can be absolved via a korban. Hashem was the only One able to prescribe teshuva as a remedy for transgressions, thusly implying that teshuva cannot be comprehended even by those most spiritually elevated.
With Reb Yonason Eibshitz's insight we can begin to understand why no one could fathom the concept of teshuva. Chazal tell us that fifty gates of wisdom were created, but even Moshe Rabbeinu was only able to reach the forty-ninth gate. The fiftieth gate, explains the Gr"a, is the gate which contains the secret of life itself, and since all mortals are destined to die, there is no way humanly possible to access the wisdom of this gate. It follows that if teshuva bears semblance to resurrection and the giving of life, then it also is beyond any comprehension and, therefore, was not entertained as a viable avenue for the rectification of sin.
This also explains one of the tefillos of Yom Kippur. We say, "Until the day of his death You wait for him; if he repents You will accept him." How can the teshuva on one's deathbed be accepted when his entire life was wasted? The answer is that teshuva is life, and it is worthwhile for a person to be in this world for an entire lifetime if he will ultimately reach a moment of clarity on his deathbed and merit tapping into true life!
However, "Praiseworthy is one who does teshuva earlier on in life" (Avodah Zara 19a). Such a person can live the rest of his years building on the foundation of true life attained through his teshuva. Teshuva is awesome and, as Rabbeinu Yonah writes, any amount of teshuva is accepted. May we be zoche to utilize the amazing opportunity that the Aseres Yemei Teshuva and Yom Kippur afford us, and begin living life in its truest form!
May we all be zoche to be mikabal ol Malchus Shamayim
and to a Gmar V'chasima Tova!