charlesatan
Explorer
That would be nice if the opponent got a penalty from the mark, but unlike the fighters combat challenge and the paladins divine challenge, no where does it say that a clerics mark imposes a penalty on the opponent. The only thing it does is remove a previous mark already on the opponent from a fighter or paladin. I suppose that's a good thing if it's the fighter or paladin who need the healing.
Check p.277 of the PHB. Marked creatures get a -2 penalty to attacks on creatures that didn't mark them.