Above the Blacksmith Andrei, there is a small chapel with a podium or altar. (I'm not talking about the chapel with the Balder Knights and the Fire Keeper Soul.) To the right of that altar there is a wall with a crack in it. When I look through that crack, there appears to be an item of some sort on the other side. I haven't been able to break the wall with any of my attacks. Is it possible to break it? If so, how? If not, is there an alternate way to get to that item? What is the item on the other side?
I'm not sure what item you're seeing through that gap, or if it's even an item at all. There isn't an accessible area behind that wall.
It's possible you're seeing the Knight Shield, which is in that direction, but is accessible by following the path back towards the chapel with the balder knights and going left before the chapel entrance. (See map here: http://darksoulswiki.wikispaces.com/file/view/Undead_Parish_Map.jpg/269510574/Undead_Parish_Map.jpg)
Maybe try picking up the Knight Sheild first and then seeing if you can still see the item through the crack... though I think if you're not seeing the shield, you're probably seeing part of the background (ie. the sun) or some kind of graphical glitch, rather than an actual item.