"Laila ha illAllah, Muhammad-ur-Rasulullah." ( There is no god except Allah; Muhammad is His Messanger).
The first part of the kalimah presents the concept of Tawheed (Oneness of God) and the second part affirms the Prophethood of Muhammad (peace be upon Him).
In order to be a Muslim-i.e. to surrender oneself to God-it is necessary to believe in the oneness of God, in the sense of His being the only creator, preserver, nourisher etc. But this belief-later on called Tawheed ar-rubu-biy-ya is not enough. Many of the idolaters knew and believed that the only supreeme God could do all these. But that was not enough to make them a Muslims. to Tawheed ar-rubu-biy-ya, one must acknowledge the fact that it is God alone who deserves to be worshipped, and thus abstain from worshipping any other thing or being whether it is an idols, angels, prophets or saints.. Having achieved knowledge of one true God, man should constantly have faith in Him, and should allow nothing to induce hin to deny his truth.
When faith enters a person's heart, it causes certain mental states which result in certain actions. Taken together, these mental states and actions are proof for true faith. The Prophets said, "Faith is that which resides firmly in the heart and which is proved by deeds."
Foremost among those mental states is the feeling of gratitude toward God, which would be said to be the essence or Ibada (worshipping or serving God)