In the fourth chapter of Hilchot Teshuva (Halacha 4; listen to audio for precise citation), the Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204) lists five sins for which it is particularly difficult to repent. The reason, he explains, is that they are not generally looked upon them as sinful. Instead, people tend to justify these actions and view them as entirely acceptable. Teshuva, of course, requires an acknowledgment that one has acted improperly, and thus repentance for these five sins while certainly not impossible is especially difficult.
The Rambam lists the five sins as follows:
1) "Ha'ochel Mi'se'uda She'ena Mesapeket Le'be'aleha" "eating from a meal that does not suffice for its host." This refers to a case of a person of meager means who invites a person to his meal despite his very limited supply of food. Although he indeed invited the guest to join, he did so only out of embarrassment, and not wholeheartedly. As such, accepting the invitation and partaking of the food amounts to a type of Gezel (theft). The guest would likely assume that he does nothing wrong by joining the meal, as the host had explicitly extended an invitation, but in truth, he has indeed committed a sin by partaking of the poor man's limited supply of food.
2) Making personal use of an item taken as collateral. If a lender takes a shovel, for example, as collateral from the borrower, he might intuitively take the liberty of using it in his garden, as this causes no damage to the tool. Nevertheless, Halacha forbids using somebody else's belongings without receiving permission, and thus making use of collateral constitutes a sin.
3) Looking upon Arayot (sights involving immorality). One might instinctively think that so long as he did not engage in forbidden relations or even have any physical contact with the person in question, he has not committed any sin. In truth, however, merely looking upon Arayot itself constitutes a grievous sin, as it can often lead to promiscuous behavior.
4) "Mitkabed Be'kelon Havero" receiving honor at the expense of one's fellow's feelings, such as gloating over correctly answering a question in a class after somebody else had given the wrong answer. A person might feel that this is legitimate since he did, after all, give the correct answer and he thus has the right to take pride even at his classmate's expense. He might not realize that the Torah strictly forbids enjoying honor and prestige that results from another person's failure.
5) "Hoshed Be'keshereim" wrongly suspecting an innocent person. People often assume that so long as they did not actually cause any harm, they have not committed any offense by suspecting people in their minds. They tend not to realize that even harboring accusations in one's mind entails a sin, regardless of whether these silent allegations are ever expressed through action.
In all these cases, the perpetrator is often unaware that he has committed a sin, and thus Teshuva for these offenses is rare. It therefore behooves us to be mindful of these transgressions so as to ensure to avoid them and that if, God forbid, we do commit such an act, we are aware of our wrongdoing and perform proper Teshuva.
