Muslim expresses his/her faith (Iman) in the following words:
Ash-hadu anla ilaha illal-Lahu Wahdahu la Sharika Lahu wa-ash-hadu anna Muhammadan abduhu wa rasuluhu
The English translation of the Shahadah is as follows:
I bear witness that there is none worthy of worship except Allah, the One, without any partner. And I bear witness that Muhammad is His servant and His Messenger.
The declaration of faith consists of two distinct parts, i.e., the absolute belief in the Unity of Allah (God) and the belief in the Holy Prophet Muhammad as a mortal human being and the Messenger of Allah.
Islam's fundamental belief is the Unity of God. All other beliefs hang on this belief. Therefore the acknowledgement and 'bearing witness' of this is the key to the Muslim faith. Unless this is observed one cannot be a Muslim. A Muslim accepts Allah as the only God, and only Master, Lord and Ruler with no partner sharing in any way His Being, Powers and Attributes. He is One; He is Unique; He is not the father of any one, nor He has or had any father. He is Almighty and Self-Sustaining. He is there forever, and will be there forever.
A Muslim believes in Allah as the Ruler and must not disobey Him. In fact everything that exists in the universe obeys Him. He is the Fashioner of all the Universes that may exist.
The belief in the Prophet Muhammad (on whom be peace) as the servant and Messenger of Allah completes one’s faith. The Holy Prophet was the greatest of all the Prophets of Allah, yet he was only a human being with no share in Divinity. This is a very important belief that keeps Muslims from associating partners with Allah. This part of the Declaration of Faith where reinforces the first part of Allah being the only Supreme Being with no partners also establishes the absolute authority of the Holy Prophet as the Messenger of Allah, who must be obeyed as well in all the matters of faith. The Holy Prophet is the last prophet who brought us the last Book (the Holy Quran) to be followed. He lived among his people for a long time and his life is documented in utmost detail. He lead his followers by example. The way he lived his life and admonished his followers to live their lives is called Sunnah of the Holy Prophet. The second part of the Declaration of Faith makes obeying and following the advice of the Prophet Muhammad equally important to the belief in absolute Unity of Allah. This declaration make one to completely submit to the Will of Allah and that is why one who declares this belief is called a Muslim - one who completely submits to the will of Allah.