Forgiveness of all sins automatically by request of forgiveness is a very distinctly Christian concept, however let us consider the key ideals and advantages such a system of morality provides us from a more secular perspective.
Given that there is such a thing as a good and such a thing as evil, whatsoever and howsoever they may be differently defined by different people, and upon disagreement of this first premise please regard a later post titled "The case for Good and Evil", we have the concept of justice implicit. For else concepts of good and evil are actionless and powerless labels of no significance at all.
Then by this we have that an evil person ought be punished and a good person not.
Take careful note however, that it is the person we punish, for it is impossible to punish the act of the person and it is only plausible to punish the person *for* the act.
What constitutes an evil person. Taking an Aristotelian perspective, it is the Hexis, the disposition and tendency unto, to evil. That is, a Will which delights in evil actions and thinks evil actions beneficial to itself and being.
Hence, if a will has no Hexis unto evil, it is not deserving of punishment. Now, introducing the concept of repentance in which a Will denounces a past action as evil and denies the motive behind it. No evil Will would deny an evil action by its own will, and the act of repentance is hence a sign that the Will in question is not Evil but Good and hence not deserving of punishment.
Note I do not say "say that it repents" but rather "repents" in itself and of itself. For it is typical of an evil Will to deceive and offer false repentance.
Hence and in conclusion, repentance is sufficient for complete forgiveness.
