Verse 9:28 from the Qur'an reads:
O you who believe (in Allah's Oneness and in His Messenger (Muhammad SAW)! Verily, the Mushrikun (polytheists, pagans, idolaters, disbelievers in the Oneness of Allah, and in the Message of Muhammad SAW) are Najasun (impure). So let them not come near Al-Masjid-al-Haram (at Makkah) after this year, and if you fear poverty, Allah will enrich you if He will, out of His Bounty. Surely, Allah is All-Knowing, All-Wise.
According to the idea of "Itmaam e Hujjah", it is not for man to declare another as 'kaafir'. How then can the disallowance of "non-Muslims" from entering the haram be justified today? And are we not denying them an experience of witnessing devotion and submission, to the One, at its best?