That Allah Almighty exists - is a reality about which there is no doubt; the following attests to that reality:
1. The sending of Messengers and the revelation of Books.
2. The natural disposition of man, which Allah created him upon.
3. The sound and reasoning mind.
The Natural Disposition of Man
This is often seen when man is afflicted with a disaster or calamity, or when all helpers and friends abandon him: his natural disposition impels him to ask Allah and to invoke Him to remove the calamity. The Noble Qur'an confirms this, saying that man turns to Allah to remove any calamity that afflicts him:
And when some hurt touches man, he cries to his Lord, turning to Him in repentance. [Az-Zumar 39:8]
The Qur'an even gives a clear example of a man who is on a boat. As the boat is sailing in clear skies, the ocean suddenly becomes violent and the bat rocks violently. The imminence of death puts him in a state of extreme fear, and it is at this point that he returns to his natural disposition and to his own supplicating sincerely to Him, without associating partners with Him. Allah Almighty says:
He it is Who enables you to travel through land and sea, till when you are in the ships and they sail with them with a favorable wind, and they are glad therein, then comes a stormy wind and the waves come to them from all sides, and they think that they are encircled therein, they invoke Allah, making their faith pure for Him Alone, saying: "If you deliver us from this, we shall truly be of the grateful." [Yunus 10:22]
Thus we find the innate belief of man in Allah's generosity, and no one will deny that except one who is intransigent in face of the truth.
The Sound and Reasoning Mind
The human mind, is fair and sincere, has no choice but to acknowledge Allah's existence, especially when considering this rule: "Nothingness cannot create something." Allah Almighty says:
Were they created by nothing, or were they themselves the creators? Or did they create the heavens and the earth? Nay, but they have no firm belief. [At-Tur 52:35, 36]
The conclusion of this verse is based on the following premises. "Did nothing create something?" The answer is that "nothing" doesn't create anything at all. Then follows the premise: "Did you create yourselves?" The answer is no, we didn't create ourselves. And then follows the third premise: "If you weren't created by 'nothing' and you didn't create yourselves, did you create the universe and its intricate system?" The answer: the one who cannot create himself cannot create anything else and the one who cannot benefit himself cannot benefit others. The conclusion the is this: if we were not created by "nothingness", if we didn't create ourselves, and if we didn't create the universe, then there must be a creator, and He is Allah Almighty, the Creator of all that exists – as He Almighty says:
Allah is the Creator of all things. [Az-Zumar 39:62]
By merely observing and contemplating the creation, one is shown that there is a creator, and that is why Allah ordered us to ponder, study, and contemplate His creation, for the creation indicates the Supreme and Perfect Creator. Allah Almighty says:
Verily! In the creation of the heavens and the earth, and in the alternation of night and day, and the ships which sail through the sea with that which is of use to mankind, and the water (rain) which Allah sends down from the sky and makes the earth alive therewith after its death, and the moving (living) creatures of all kinds that He has scattered therein, and in the veering of winds and clouds which are held between the sky and the earth, are indeed Ayat (proofs, evidences, signs, etc.) for people of understanding. [Al-Baqarah 2:164]
[Credit goes to 'What Must Be Known About Islam', a comprehensive book, compiled by Muhammad bin 'Ali Al-Arfaj, translated by Darussalam, global leaders in Islamic books.]