Welcome back, suskun, with your wonderful inscriptions!
Yes, you're right. This is the most difficult and unreadable one.
We only know for sure that it is Christian, because of the cross at the top.
It also seems to be a dedication, if I am right about [Α]ΝΕΘΗΚΕ ("set up," "dedicated") in the third line. If so, it is not clear who, or what, or to whom.