Position of Hajj and its merit

The Hajj in Islamic law is a group of sacred rites and is one of the pillars upon which Islam is established as it has been quoted from Imam al-Baqir (a.): "Islam has been built on five things: salat, zakat, sawm, Hajj and wilayah to the Ahlul-Bait (a.)".
  Hajj in its two parts, the wajib and the mustahabb, is of great merit and with an ample reward. Many hadiths have been quoted from the Prophet Muhammad (s.) and his progeny (a.) regarding the merit of Hajj. Imam al-Sadiq (a.) said: "The pilgrim and mu`tamir are the guests of Allah, if they question Him, He will answer them, if they supplicate to Him, He will answer them, if they want to intercede for others, He will accept it, and if they keep quiet, He will be the beginner, and they will be compensated instead of one Dirham, a million Dirhams".

The ruling regarding those who deny the obligation of Hajj and those who neglect it

The obligation of Hajj is a proven subject in the Qur'an and Sunnah and it is among the necessities prescribed in our religion. Its neglect is considered as a great sin if he who neglects it possesses the required characteristics and he knows that it is wajib. Allah the Sublime in the Qur'an said: "Pilgrimage to the House is a duty imposed upon mankind by God, for anyone who can afford a way to do so. Any one who disbelieves will find that God is Transcendent, beyond any need for the Universe". It has been quoted from Imam al-Sadiq (a.) that: "If any one died without performing the Hajj and there was no harmful request, severe illness or preventing sultan, which might have prevented him from doing it, he would have died like a Jew or a Christian".

Types of Hajj

Hajj, performed by the mukallaf, is either for himself or on behalf of another person. The second is called Hajj al-niyabi. The first is either wajib or mustahabb. The Hajj al-wajib is either originally wajib in Islamic law, which is called 'Hajjat al-Islam', or it is accidentally wajib through a nadhr or if the Hajj had been invalidated.
  For each Hajjat al-Islam and Hajj al-niyabi there are conditions, which will be mentioned in the first part within two chapters. Hajj is also divided into Hajj al-tamattu`, ifrad and qiran. The first is the duty of those whose home is located a distance of 60 farsakh (about 90 km or more) away from Mecca al-mukarramah. The second and the third are the duties of those live in Mecca or outside it within this distance.
  Hajj al-tamattu` differs from the two other kinds in its manasik and practices which would be cleared up within several chapters in the second part.