The existence of Allah Almighty is something innate in the humans' nature and not among complicated sciences or impossible to realize. Despite the fact that Allah's existence is evident everywhere through His creatures while some are heedless about it, neither a believer nor an atheist can deny His existence. Allah Almighty says,
The messengers said, "Can there be doubt about Allah, Creator of the heavens and earth?" [Qur'an 14:10].
Therefore, Allah sent the messengers to guide people to the right concept of Divinity (Uluhiya). This is because even people have realized the existence of Allah by nature; they went astray by associating partners to Him.
Allah Almighty says,
This [Qur'an] is notification for the people that they may be warned thereby and that they may know that He is but one God [Qur'an 14:52].
So know, [O Muhammad], that there is no deity except Allah and ask forgiveness for your sin [Qur'an 47: 19].
Evil environment and faith
Corrupt environment seriously affects humans' primordial good nature through abolishing it and leading them to renounce what is good and engage in evil and corruption. This is the secret why some people reject faith and righteousness and accept disbelief and polytheism which contradict the human's primordial good nature and sound thinking.
Allah Almighty says in Qudsi hadith, "Verily I created My servants Hunafa' (monotheists), but then the devils came to them and distracted them from their religion prohibiting what I have made lawful for them''.
The prevailing western civilization tend towards arousing doubt around the existence of Allah and disdaining heavenly revealed religions or limiting them as temporary social tranquilizers for its followers. It is undoubtedly that the world now suffers a spiritual calamity which springs from denying the sublime standards of the religion including truth, justice, tolerance and brotherhood. Returning back to such values is the only means for guidance and the primordial religion.
As long as people follow their innate good nature, then they are guided to Islam which is the primordial religion. There is no harm in mentioning some examples that open the mind of those who are heedless to realize the truth.