Question: A listener from Riyadh asks: “What does it mean to testify that there is no deity but Allaah and that Muhammad is the Messenger of Allaah?”
Answer: Shahaadah (Testimony of Faith) means to testify both verbally and inwardly that there is none worthy of worship except Allaah. A word uttered by the tongue, and one must also believe in one’s heart that there is no god but Allaah and none has the right to be worshiped except Allaah. It should also be believed that all other deities worshiped by people, including idols, the dead, trees, stones, angels or anything else are false deities. Allaah (Exalted be He) stated: That is because Allaah — He is the Truth (the only True God of all that exists, Who has no partners or rivals with Him), and what they (the polytheists) invoke besides Him, it is Baatil (falsehood).
This is the actual meaning of testifying that there is no deity but Allaah. In conclusion, it is a faithful and strongly-substantiated testimony that none has the right to be worshiped but Allaah and that all deities worshiped by people are false… read more here.