Testimony of Faith (Shahadah)
Shahadah is the testimony of faith that ‘none has the right to be worshipped but Allah سبحانه وتعالى and Muhammad صلى الله عليه وسلم is the messenger of Allah سبحانه وتعالى’. This testimony is the most fundamental principal of Islam and forms the driving force for submission to all the other acts of worship. Although, one can be declared as Muslim by utterance of these words by tongue, the real testimony of the faith is an act of submission by heart. The more one submits to this belief, the more he develops the love and fear of Allah سبحانه وتعالى and more valuable he becomes in the eyes of His Creator.
All the messengers of Allah سبحانه وتعالى who were sent to past nations taught the lesson of Monotheism (Oneness of Allah سبحانه وتعالى) and asked people to follow a set of rules and regulations (Shariah). Allah سبحانه وتعالى is The Only One worth worshiping and He is The Only One who has the power to decide what is right (Halal) and what is wrong (Haram). And He is The Only One who can forgive sins and decide the ultimate fate of any human being. He sent his messengers to each and every nation for their guidance. Mohammad صلى الله عليه وسلم is the last messenger of Allah سبحانه وتعالى and the message sent to him is there to guide the mankind till the Day of Judgment, in the form of The Holy Quran and The Hadith (Collection of sayings and life events of The Prophet صلى الله عليه وسلم).
Note: For any detailed knowledge about the Islamic faith and for your questions unaddressed on this website, you are advised to consult an authenticated Islamic scholar (Mufti) in your locality.