All praise is due to Allaah. May peace and blessings be upon the Last Prophet. Allaah, the Exalted, states, This day, I have perfected your religion for you, completed My Favour upon you, and have chosen for you Islaam as your religion. Allaah (Glorified be He) also says: Truly, the religion with Allaah is Islaam. Allaah (Glorified be He) also says: And whoever seeks a religion other than Islaam, it will never be accepted of him, and in the Hereafter he will be one of the losers.
Islaam is to submit to Allaah by Tawheed (belief in the Oneness of Allaah/monotheism), yielding to Him by obedience and leaving acts of Shirk (associating others with Allaah in His Divinity or worship) and those who commit Shirk. Shirk was the belief of the Arabs before the advent of the Da`wah (calling to Islaam) of Muhammad (peace be upon him). It is narrated by Al-Bukhaaree that ‘Aboo Raja’ Al-`Utarydee said, we used to worship a certain stone, but if we found a better stone, we would throw the first stone away and worship the other one. If we did not find any stone to worship, we would grasp a handful of dust, pour goat milk over it, then we would circumambulate it… read more here.