xvii INTRODUCTION
In the name of Allah, the Most Beneficent, the Most Merciful

WHY ALLAH SENT PROPHETS AND APOSTLES?

Ever since people innovated the dogma of Shirk, (i.e. joining others in worship along with Allah)(1). Allah had been sending Prophets and Apostles to His Devotees in order to invite them to the Worship of Allah and Allah Alone, to order them not to ascribe partners unto Him and bring them out of the darkness of polytheism into the light of monotheism. All the Prophets preached Tauhid (i.e. monotheism, the belief in the Oneness of Allah, the Glorious, the Elevated). The following Verses from the Holy Book (Qur'an) illustrate this fact:

"Indeed We sent Noah to his people, and he said: O my people! Worship Allah. You have no other god except Him. Verily! I fear for you the Punishment of a Great Day (7:59) And unto (the tribe of) 'Ad (We sent) their brother Hud. He said: O my people! Worship Allah. You have no other god save Him. Will you not ward off (evil)? (7:65) And unto Midian (We sent) their brother, Shu'aib. He said: O my people! Worship Allah. You have no other god save Him. (7:85) And to (the tribe of) Thamud (We sent) their brother Salih. He said: O my people! Worship Allah. You have no other god save Him. (7:73) Verily We have raised in every nation an apostle (proclaiming): Worship Allah and shun false gods. (An-Nahl, 16:36).

Every prophet was sent unto his own nation for their guidance, but the message of the Prophet Muhammad was general for all mankind and Jinns. In Surat-al-A'raf Allah addresses His Apostle

"Say: O mankind: Verily! I am the Apostle of Allah to you all." (7:158).

Dear reader, if you think of the period nearly 560 years after Prophet Jesus was lifted, up by Allah, the Glorious, the Elevated, you will find that the people had left the
(1) Shirk Opposite of Tauhid and it is to worship others along with Allah. 

Tauhid is of four aspects.
(a) Unity of Lordship. "Tauhid-al-Rabubiya", to believe that there is only one Lord for all the Universe, and that is Allah.
(b) Unity of Worship: "Tauhid-al-Uluhiya", to believe that none has the right to be worshipped but Allah.
(c) Unity of the Names and Qualities (Attributes) of Allah: "Tauhid-al-Asma: was Sifat", to believe that
(1) we must not name or qualify Allah except with what He and His Apostle has named or qualified Him.
(2) none can be named or qualified with the Names or Qualifications of Allah, e.g. Al-Karim.
(3) we must confirm all of Allah's Qualifications which Allah has stated in His Book (Holy Qur'an) or mentioned through His Apostle Muhammad (P.B.U.H.) without twisting the meaning and giving resemblance to any of the created things. These three aspects of Tauhid are included in the meaning, "None has the right to be worshipped but Allah."
(d) Unity of following Allah's Apostle (P.B.U.H.) "Tauhid-al-Ittiba", and this is included in the meaning of: I testify that Muhammad (P.B.U.H.) is Allah's Apostle and that means: None has the right to be followed after Allah's Book (orders) i.e. Qur'an but Allah's Apostle (P.B.U.H.)