The Amulet of the Devout requires attunement by a Cleric or Paladin, and provides the following benefit:
This amulet bears the symbol of a deity inlaid with precious stones or metals. While you wear the holy symbol, you gain a +1 bonus to spell attack rolls and the saving throw DCs of your spells.
Suppose I have a multiclassed character that can attune to the amulet, such as a Paladin / Sorcerer. Does the bonus of the amulet apply to all my spells, or only to my Cleric & Paladin spells?
The item states:
While you wear the holy symbol, you gain a +1 bonus to spell attack rolls and the saving throw DCs of your spells.
It does not restrict this bonus to a certain class of spells; it does not say "your paladin spells" or "your cleric spells" but simply says "your spells" and because of this, it applies to all of your spells, no matter what class they come from.
Two examples of items that do have restrictions based on class are the Rod of the Pact Keeper or the Book of Exalted Deeds:
While holding this rod, you gain a +1 bonus to spell attack rolls and to the saving throw DCs of your warlock spells. [...]
[...] any spell slot you expend to cast a cleric or paladin spell counts as a spell slot of one level higher. [...]
The Amulet of the Devout does not use wording similar to these items and so it does not have the same sort of restriction; it benefits all of your spells.