Purpose of Life and the Prophets
In Islam everyone is born pure without sin (and as a muslim). God has given each soul a conscience (the knowledge of what is right and wrong), and free will.
The prophet Mohammed is known to have said,
Righteousness is composed of good behavior. And sin is what you don't want people to know, despite the fact that it eats away at your heart (Muslim, Birr 14-15)
This short transitory life is then simply a test within ourselves to recognise and praise God, and outweigh our righteous actions over our bad. Thus proving worthy on the on the Day of Judgement in front of Almighty God. To ultimately gain entry to paradise to live forever in bliss.
To reinforce this and show the clear way to worship God, prophets were sent as a guide to the right path.
"Say: We believe in Allah (God) and that which is revealed to us, and in what was revealed to Abraham, Ishma'il, Isaac, Jacob, and the tribes, to Moses and Jesus and the other prophets from their Lord. We make no distinction between any of them, and to Allah we have surrendered ourselves." (Qur'an, 2:136)
"It is not devotion to turn your faces towards the East or the West. Rather, true devotion is believing in Allah and the Last Day, and the Angels, and the Book, and the prophets. (Quran 2:177)
In addition: "Anyone who rejects Allah and His angels and His Messengers and the Last Day has gone very far astray." (Quran 4:136)
Mohammed (P.B.U.H), was the last of the prophets, who reaffirmed the same message, removing any distortions that may have crept in over time.
Present day Sources of Guidance
As no more prophets are to come God has left two sources of guidance:
a) The holy Quran, the final testament.
A summation of all the holy books of the past. The literal word of Allah, which Allah has promised to keep unchanged. This challenge by the Almighty is witnessed by the oldest Qurans being exactly the same as the newly printed ones 1400 years later. One billion muslims around the globe read the same one Quran.
b) The Hadiths (sayings and actions of the Prophet).
The prophet Muhammad (P.B.U.H.) was the living interpretation of the Quran, his every action and saying was a mirror reflection of the Quran with the guidance of Allah(swt). So for muslims, and non-muslims, the study of the Quran and Hadiths is of utmost importance. Preferebly with the aid of an Islamic scholar as a guide to gain a deeper understanding.