Protection from Evil is a buff that produces several effects on both the protected creature and others that interact with them.
First, it causes the protected creatured to be completely ignored by summoned demons, so the character will never be attacked/targeted, but can still, say, be harmed as a collateral effect of a demon's Fireball. Keep in mind that this demon-protection effect doesn't work against all demons; only those specifically summoned by spells like Cacofiend, Summon Fiend, and Gate; as well as, uniquely, the Glabrezu in the Guarded Compound. Other demons, like Aec'Letec, simply spawn like ordinary creatures and the spell doesn't prevent them from targeting the protected creature. The effect will still apply its attack roll and Saving Throw effects normally to and against these demons, however.
Second, this effect provides the Saving Throw/attack roll bonuses/penalties described in the tables below - that vary from game to game.
In the original games, this effect is also cumulative with itself; but, in the Enhanced Editions, it cannot be stacked - even if coming from different sources.
All Protection from Evil effects can be removed by Dispel Magic effects. And a Breach will remove them as well.
The actual effects of the Protection from Evil buff were changed throughout the course of the Baldur's Gate series which was not always reflected in spell descriptions.
Baldur's Gate[]
| Armor Class: | -2 bonus |
| Saving throws: | -2 bonus |
| Immunities: | Charmed |
Besides using a fixed duration for its Protection from Evil spells, the very first game applies a general Armor Class and Saving throw bonus that doesn't check the alignment of enemy creatures. As summoned demons do not exist in the original Baldur's Gate, the later added effect isn't needed – instead it is also the only game that simulates the actual "second" pen-and-paper effect (attack and save bonuses are pooled as first there): immunity to becoming possessed by evil creatures, read "charmed". This, however, isn't written into the description.
Shadows of Amn[]
| Situational AC: | 2-point attack roll penalty to evil attackers (friend or foe) |
| Immunities: | Summoned demons |
The original Shadows of Amn doesn't alter any of the Protection from Evil spells' descriptions (besides updating their durations, as needed) but removes the saving throw bonus (though this requires further tests to be confirmed) and the charm immunity. The replacement effect, protection from summoned demons, is not mentioned.
Enhanced Editions[]
| Situational AC: | 2-point attack roll penalty to evil attackers (friend or foe) |
| Saving throw: | 2-point bonus against evil creatures (friend or foe, including the caster's own spells) |
| Immunities: | Summoned demons |