From that which has been established in the Sharee'ah (prescribed law) is that mankind was - in the beginning - a single nation upn true Tawheed, then Shirk (directing any part or form of worship, or anything else that is solely the right of Allaah, to other than Allaah) gradually overcame them. The basis for this is the saying of Allaah - the Most Blessed, the Most High:
"Mankind was one Ummah, then Allaah sent prophets bringing good news and warnings." (Soorah Baqarah 2:213)
Indeed all praise is for Allah. We praise Him; seek His aid; seek his forgiveness; and seek refuge with Allah from the evil prompting of our souls and the bad consequences of our actions. Whomsoever Allah guides, there is none to set him astray; and whomsoever Allah sets astray, there is no guide for him. And I testify that none has the right to be worshipped but Allah alone, He has no partner; and I testify that Muhammad is His Slave and His Messenger.
by Imam Ibn Qayyim al-Jawziyyah
Islam is based on five pillars which the Messenger (peace and blessings of Allaah be upon him) has explained for us, when he said: “Islam is built upon five [pillars]: the testimony that there is no god but Allaah and that Muhammad is the Messenger of Allaah; establishing regular prayer (salaah); paying the Zakah; Hajj (pilgrimage) and fasting Ramadaan.” (Agreed upon; narrated by al-Bukhaari, no. 8)
The Meaning of Our Testimony that Muhammad (P.B.U.H) is the Messenger of Allah