Question: Is a sin different from shirk?
ANSWER
Islamic books list shirk among major sins. As a matter of fact, it is declared in a hadith-i sharif:
(Major sins are as follows: to attribute a partner to Allah, to be disobedient to one's parents, and to take a false oath.) [Bukhari]

[Shirk means to attribute partners to Allahu ta’ala. A person who does the attributing is called a mushrik (polytheist), and the thing which is attributed is called a sharik (partner). To believe that someone possesses one of the attributes of Divinity means to make him a partner. The attributes possessed exclusively by Allahu ta’ala are called Attributes of Divinity.]

Another hadith-i sharif says, "Sins are of three types: sins that will not be forgiven on the Day of Resurrection, sins that are not discontinued, and sins that will be forgiven if Allahu ta'ala wishes." The sins that will absolutely not be forgiven on the Day of Resurrection are shirk. In addition to them, the Qur'an al-karim says, "Allahu ta'ala forgives sins except shirk if He wills" (An-Nisa' 48).

The sins that are discontinued are the sins that involve violation of others' rights. The sins that will be forgiven if Allahu ta'ala wishes are the sins that do not involve violation of others' rights. (Bariqa)

When it is said, "He who commits sins does not become a disbeliever," it should be understood that the sins mentioned here do not include shirk.

Riya is the hidden shirk. [Riya means to present something in a manner opposite to its true nature. In short, it means pretension, i.e., a person’s performing deeds for the next world to impress the idea on others that he is really a pious person with earnest desire of the next world while in fact he wants to attain his worldly desires and the wealth of this world. In other words, it means to use the religion as a tool to obtain worldly riches or to bring oneself into other people’s favor by making a show of one’s acts of worship.] Allahu ta'ala says about a person who adulterates his deeds with riya, "So-and-so is making fun of Me." The case is like that of a young man who wants to marry the daughter of an imam. In order to ingratiate himself with the imam, he offers salat in the first row and serves him. His motive is not to serve the imam but to prepare the ground for marrying his daughter.

Similarly, a person who performs acts of worship with riya wants to win a place in the hearts of people. This means making mockery of Allahu ta'ala, which is a major sin. For this reason, our Master the Prophet declared:

(Riya is the minor shirk.) [Tabarani]

(Beware of shirk. It is more hidden than the footsteps of an ant.) [Hakim]

If a person, while performing salat, is more meticulous about it when he is in company, he is considered to have shown reverence to someone other than Allah. This act is close to shirk. Though outwardly he is revering Allah, inwardly he is revering people. Hadrat Ibn Mas'ud said, "They are considered to have taken Allah lightly," to mean such people. The similitude of a person who performs acts of worship with great care when he is with his acquaintances is an idiot who sits one leg over the other in the presence of a ruler but takes it down and observes manners in the presence of the servants of the ruler.

Our Master the Prophet ordered us to stay away from shirk, which is more inconspicuous than the footsteps of an ant. This is such a riya that not everybody can sense it easily. For example, a person offers his acts of worship in solitude and does not want to inform anyone about them. He does not feel happy either even if people come to hear about his acts of worship. However, he wants others to respect him owing to his acts of worship and the services he rendered to Islam. But for his services, he would not expect respect and love from anybody.

Though a person is performing his acts of worship solely for the sake of Allah, it is hidden shirk to want others to esteem him because of his acts of worship. Hadrat Ali reports that Allahu ta'ala will say the following about such kinds of people on the Day of Resurrection:
"Were things not made easier for you in buying and selling in worldly life? Were your needs not met and favors not done to you? You received the rewards for your deeds during worldly life. Now there is no reward for you."

It is a symptom that is indicative of riya to be glad when one's acts of worship and good deeds are heard about and commended by people. Once, a person said, "I read Surah al-Baqarah at night." Thereupon, Hadrat Ibn Mas'ud stated, "As you disclosed it, you have received the reward for what you read in the world."

When another person said, "O the Messenger of Allah, I fasted all the year round," he answered, "You are not considered to have fasted" (Muslim).

We should refrain from making known our good deeds. A hadith-i sharif says:
(If a person endeavors to make his deeds public and does riya, he will remain in the wrath of Allahu ta'ala until he forsakes it.) [Tabarani]

Of all the sins and vices, disbelief (kufr) is the worst. There are many types of disbelief. The worst of all is shirk. Any variety of a certain vice is mostly specified with its most outstanding characteristic. Therefore, the word shirk used in Qur'anic verses and hadith-i sharifs represent all sorts of kufr (disbelief).

If a person who has attributed partners to Allah repents of it, his repentance is accepted. However, Allahu ta'ala does not forgive a person who has died a mushrik without making repentance. If a mushrik makes repentance before dying, he is forgiven. (Radd-ul-mukhtar)

Besides, when a disbeliever becomes a Muslim, all his sins are changed into thawab. Allahu ta'ala's forgiveness and mercy is vast. As soon as a person embraces Islam and does good deeds, He transforms his sins into thawab. Everything belongs to Him, so He gives whatever He wishes to whomever He wishes as much as He wishes. There is nothing to be confused about this.

The Qur'an al-karim says (what means):
(Those who repent, believe, and do good deeds, I will transform their sins into thawab. Allah is very forgiving, very protective, and very merciful.) [Al-Furqan 70]

It is written in the book Hadiqa that this verse was revealed for Hadrat Wahshi. Tafsir al-Madarik says that what is meant by "who repent" is "who repent of shirk." As is seen, the sins of those who repent of shirk and believe are replaced with thawab.