The first sentence of the Islamic kalimah,i.e. "There is no god except Allah," leads a Muslim throughout his life not only in religious matters but in social behaviour also. "There is no god" shows a Muslim that nothing in the Universe is superior to him. It is observed in Qur'an that:
"He it is Who created for you all that is in the earth ."(2:29).
So a Muslim knows that nothing in this world is to be worshipped. Neither stone nor trees; neither animals nor human-beings; neither the Sun nor the Moon nor the Stars can be worshipped; because everything is created, and created for his benefit. When a Muslim, thus, has rejected every falsehood and every idea of Nature-worship, Idol-worship or Human-worship, he is ready to believe in the positive truth of the Unity of God. Believing in a Supreme being gives an aim to our life and provides a purpose for our actions. Had a man been left with the wrong impression that there was no God at all, his life would have been aimless, and an aimless life is dangerous. So it is added that there is no god "except Allah." This sentence has a negative as well as a positive aspect. Both are instrumental in creating the belief that every man is equal to every other person. When nobody is superior, nobody is inferior. Thus, the belief in the Unity of God promotes the sense of brotherhood and equality and equity which is another feature of Islam.
Now allow me to explain other aspects about the belief in the Unity of God:
We believe that God is Omnipotent and Omniscient. He was there when there was nothing. Nothing except Him is eternal. He has no beginning and no end. He knows everything. Even our unspoken intentions and desires are not hidden from Him. We believe that He has no partner, no advisor, no helper. He has no son, no daughter, no wife. He is above every need, because need presumes deficiency. He has no body, because body needs space. He is nowhere, because He has no body; and He is everywhere, because His power and knowledge is magnificently apparent everywhere. He cannot be defined in terms of Time and Space. He cannot be incarnated, because incarnation needs body and space. He cannot be seen because of the same reason.
When we say that Allah is Omnipotent and Merciful, we do not mean that His Power and Mercy is something different from His Person. We see that a child is born without any power, and then he acquires strength day by day. It is so because power is not his person. Allah is not like this. He is Power Himself; Mercy Himself; Knowledge Himself; Justice Himself; Virtue Himself; Truth Himself and so on.
The proper name which we use for God is 'ALLAH' and that also needs some explanations.
'ALLAH' means `One who deserves to be loved' and `Into Whom every one seeks refuge.' This word, grammatically speaking, is unique. It has no plural and no feminine. So this name itself reflects light upon the fact that Allah is one and only one; He has neither any partner nor any equal. The name cannot be translated by the word `GOD' because God can be transformed into `gods' and `goddess.'