*Puts on protective gear*
There is no way of arguing around this - we are going to nerf a spell - divine favor.
The change is done mainly because the spell has proven excessively powerful at higher levels, allowing +5 attack bonus at level 15 that allows penetrating more or less all DR, while only being a level 1 spell.
What we will do is change the spell to cap at +3. This is what was done in WotC's own update to the 3.5 rules, which confirms the conception that the spell is too powerful for it's level.
Sorry for all the negative feelings this may invoke.