Testify
Testify without witnessing

You can never be a witness for what you haven't witnessed with your own eyes and this is very simple according to the general rules and regulation of the world. Rather, you can Believe in the incident having trust the source. But the Muslims of the world testify about the God while none of them has ever seen Him let alone witnessed though it's a fact that God exists undoubtedly. To realize this topic we can see an example. Let's explore that;

Suppose, John went to the market and there he witnessed a murder incident by his own eyes. Getting back to William, John describes the incident to him and William strongly believes it because he knows John very well. 

Now, a question arises. If the court demands witness, can William be a witness to the court to testify about the murder? Can William testify regarding the murder he heard and believed? The answer is simply, "No". Because, he has not seen it by his own eyes. This the reality. In the same way, Allah said in the holy Quran to have faith in Him, His prophet and so on but never said to testify His existence and power. Then why do the Muslims say, "I testify that there is no God but Allah" while they haven't seen Allah by their own eyes?