Faith is a magic spell in Final Fantasy VII Rebirth. It is the second spell provided by 
Empowerment Materia. It grants the 
Faith effect, which boosts the 
Magic Attack of the _target, thus boosting their magic damage output.
The spell is opposed to 
Defaith, which inflicts the 
Defaith effect that reduces magic attributes. As such, casting it on a _target with defaith will remove defaith rather than apply faith. The spell also has a physical equivalent, Bravery.
It is also used as an enemy ability by Two Face.
Use
Faith costs 7 MP to cast. It grants a supportive status that boosts magic damage attributes for its duration. The spell can be an effective way to boost the damage output of a party member who relies on magic damage, such as Aerith, Yuffie, or Cait Sith or Cloud in certain builds.
The spell retains use throughout the game particularly on a party member with 
Genji Gloves. This is because their damage potential will be unlimited, meaning casting Faith can push their damage further than their attributes normally grants them. On Aerith or Yuffie, whose Arcane Ward and Doppelganger allow them to cast spells multiple times, this can grant them the ability to deal immense amounts of damage in a short space of time.
Because faith is opposed to defaith, casting faith will remove a defaith effect but not apply faith to replace it. This has a cheaper MP cost than Esuna, unless one wishes to remove multiple statuses at once.
Faith can be cast on all party members at once when empowerment materia is paired with 
Magnify Materia to apply to the party. However, Faith is better when _targeted toward on one party member anyway, as one member is more likely to be the source of magic damage than an entire party. Should a player wish to boost an entire party's magic damage output, it is easier to simply cast Deshell on an enemy.
An alternative to using Faith is using Sonic Boom from the 
Enemy Skill Materia.