Allahu ta’ala may avenge for any sin which has not been repented for. For, Allah’s wrath is hidden in sins. Allahu ta’ala is very very powerful, victorious over everybody, and He is vindictive. He may refuse eternally a liked slave of His who has been worshipping for a hundred thousand years for one single sin, nor does He fear anything. The Qur’an informs of this fact and communicates that the Devil [Satan], who had been obeying Him for two hundred thousand years, became eternally accursed because he was too arrogant to prostrate. A son of Hadrat Adam’s, His representative on the earth, was eternally expelled by Him because he killed a man.
Bal’am-i Baura, during the time of Hadrat Musa, knew the Ism-i a’zam. Every prayer of his would be accepted (by Allahu ta’ala). His knowledge and worshipping were in such a high degree that two thousand people would be present with him with their pens and ink pots in order to write what he said and benefit from his words. This Bal’am, showing a little inclination towards one haram of Allah’s, died without iman (faith). He became the subject of public talk which said, “Those who are like him are like dogs.”
Qarun was a relative of Hadrat Musa’s. Having been blessed with the benedictions of Hadrat Musa, and being taught the knowledge of chemistry by him, he had become so rich that only the keys of his treasuries would be carried by forty mules. Because he did not pay the zakat (alms giving), which was worth a few cents, he was made to go under the earth together with his entire property. Sa’laba, among the Sahaba , was very zahid (pious). He used to worship very much, so much so that he would not go out of the mosque. Because of one breach of promise he lost his chance to attain the grade of being Sahabi, and died without iman. Our Master, the Prophet, was commanded not to invoke any blessings on him. Allahu ta’ala has avenged upon many other people like these on account of one sin. Then each Believer should be very much afraid of committing sins. Upon committing one small sin he should repent, say istighfar (repentance) and beg for forgiveness.”
 Sahaba: if a Muslim has seen the Prophet, or talked to him, at least once when the Prophet was alive, he is called Sahabi. Plural form of Sahabi is Sahaba or As’hab. The word Sahaba-i kiram includes all those great people each of whom has seen the Prophet at least once. The lowest of the Sahaba is much higher than the highest of other Muslims. If a person has not seen the Prophet but has seen or talked to one of the Sahaba at least once, he is called Tabi’. Its plural form is Tabi’in. In other words, the Tabi’in are the successors of the Sahaba. If a person has not seen any of the Sahaba but has seen at least one of the Tabi’in, he is called Taba’i Tabi’in. The Sahaba, the Tabi’in and the Taba’i tabi’in altogether are called the Salaf-i salihin (the early savants).