Islam is not a new Religion started by the Prophet Muhammad(peace be upon him) around 1400 years ago.It is the same truth proclaimed by all the Prophets who came before him, right from the time of Adam.Muslims believe in all the Prophets like (to name a few)Abraham,Moses and Jesus(peace be upon them all) and the divine revelations like the Torah(Jewish Holy Book) and the Bible(Christian Holy Book). But these books have been interpolated by so-called "saints" or "holy men" over a period of time to suit their whims and fancies for example, Pork is prohibited in the Torah and the Bible but these have been iterpolated and made into acceptable things over a period of time.
The Holy Qur'an clearly states-
This Qur'an is not such as can be produced by other than God; On the Contrary it is a Confirmation of (Revelation) that went before it. And a fuller explanation of the Book Wherein there is no doubt from the Lord of the Worlds. Holy Qur'an (Chapter 10, Verse 37)