The Believers Attitude

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 Tawhid Ar-Rububiyyah is not enough. Many of the idolaters knew and believed that only the supreme God could do all this. But that was not enough to make them Muslims. To Tawhid ar-rububiyyah one must add tawhid al'uluhiyyah i.e., one acknowledges the fact that it is God alone who deserves to be worshipped, and thus abstains from worshipping any other thing or being.
Having achieved this knowledge of the one true God, man should constantly have faith in Him, and should allow nothing to induce him deny 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 the proof for true faith. The prophet 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 towards God, which could be said to be the essence of Ibada (Worship).
The feeling of gratitude is do important that a non-believer is called "kafir", which means "one who denies a truth" and also "one who is ungrateful".
A believer loves, and is grateful to God for the bounties He bestowed upon him, but being aware of the fact that his good deeds, whether mental or physical, are far from being commensurate with Divine favors, he is always anxious lest God should punish him, here or in the Hereafter. He, therefore, fears Him, surrenders himself to him and serves Him with great humility. One cannot be in such a mental state without being almost all the time mindful of God. Remembering God is thus the life force of faith, without which it fades and withers away.