Chivalry is exclusively used by the Crusader. It is required for the Crusader Manifesto, the crusader spellbook which include healing, curing of poison, improved strength, and holy magic damage.
The Manifesto can be bought at the Crusader Trainer at Barra's Outpost.
Casting chivalry spells from the Manifesto require tithing points. If you are casting from the scroll, you do not require any tithing points. In addition to tithing points, spells also require mana and reagents. Their rate of success is determined by the Chivalry skill while the power and duration is based on your amount of virtue.
Tithing points can be bought from a chivalry shrine. There are only two shrines available: one at Barra's Outpost and the other at the Asgard Cathedral. Tithing points cost 10gp per point. You can have up to 1000 tithing points at any time. To see how many tithing points you have, either use the shrine or check the Manifesto.