Humans have flawed systems of perception and attention. It is little wonder that many of the leaders in the skeptical / freethought movement are current or former magicians (e.g., James Randi, Penn Jillette & Teller, Richard Wiseman, D.J. Grothe, etc.), because magicians capitalize on the vulnerabilities of human perception and attention.
There are several examples of ways our senses can be fooled, but optical illusions are perhaps the easiest to display on the internet. Here are three of examples:
(The horizontal lines in the picture above are perfectly straight & parallel.)
(In this second picture, there is no actual movement occurring.)
(In the picture above, Square A & Square B are exactly the same shade of gray.)
The reason that we know these are illusions is that we can test them with instruments (sometimes with one as simple as a ruler), but if we were dependent ONLY on our eyes, most people would assume that the illusory experience is true.
Christian/religious experiences seem true to the believers, but not to the skeptics, and any efforts by the believers to offer proof sound silly, just like trying to convince someone that Square A really is darker than B simply because our eyes say so.
Since human senses can be fooled so readily, since Christians have no instrument to test religious experiences, and since such experiences fall so utterly far outside the norm of daily life, it appears reasonable to conclude that such experience can be explained more easily as misperception & misjudgment of a real experience, just like a mirage can be seen as an oasis in the desert.
(By the way, if you still think Square A is darker, click here.)