[19].Surah Maryam [Mary] : Ayat 27-28

Ayat 27. Then she brought him (the baby) to her people, carrying him. They said: "O Mary! Indeed you have brought a thing Fariya (an unheard mighty thing, unlawful sin).

Ayat 28. "O sister (i.e. the like) of Harun (Aaron) [not the brother of Mûsa (Moses), but he was another pious man at the time of Maryam (Mary)]! Your father was not a man who used to commit adultery, nor your mother was an unchaste woman."

[Tafseer] The amazement of the people knew no bounds. In any case they were ready to think the worst of her, as she disappeared from her kin for some time. But now she comes, shamelessly parading a babe in her arms! How she had disgraced the house of Aaron, the fountain of priesthood! We may suppose that the scene took place in the Temple in Jerusalem , or in Nazareth (Cf. 4:156). Mary and her cousin Elisabeth (mother of Yahya) came of a priestly family, and were therefore "sisters of Aaron' or daughters of Imran (who was Aaron's father). Mary is reminded of her high lineage and the exceptional morals of her father and mother. How, they said, she had fallen, and disgraced the name of her progenitors! "  

Hazrat Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah [SAWW](PBUH) said, "Keep away from the seven fatalities.'' It was asked: "What are they, O Messenger of Allah?'' He [SAWW](PBUH) replied, "Associating anything with Allah in worship (i.e., committing an act of Shirk), sorcery, killing of one whom Allah has declared inviolable without a just cause, devouring the property of an orphan, the eating of usury (Riba), fleeing from the battlefield and accusing chaste believing women, who never even think of anything touching their chastity.''

[Al-Bukhari Book 04,Chapter 51, Hadith # 028] 

Lesson : as mentioned above in Surah Maryam Ayat 27."They said: "O Mary! Indeed you have brought a thing Fariya (an unheard mighty thing, unlawful sin)" All the sins mentioned in the Hadith are major sins but Shirk is the greatest of all. The reason behind this is that it will never be pardoned, and one who commits it, will abide in Hell for all eternity. So far as the other major sins are concerned with Allah, He may forgive them if He so wills; otherwise one who is guilty of such sins will have to suffer the torments of Hell. He will, however, be sent to Jannah after suffering the punishment if he is a Muslim.