Fervor in Crusader Kings 3 is basically a measure of how righteous adherents believe in a certain Faith or Religion. The range goes from 0% to 100%. Low Fervor makes it easier for Rulers to create new faiths of their own and increase the likelihood of Counties and characters converting to other faiths, including Heresies. High Fervor increases County conversion speed.
Fervor changes naturally with time. It is normal for smaller Faiths to have higher Fervor and larger Faiths to have low Fervor. Waging Great Holy Wars and successful Holy Wars will decrease Fervor. Being the target will increase Fervor. Outbreaks of Heresy will increase Fervor significantly.
To view the Fervor of the Faith you follow, click or hover over the Faith icon to the right of your character in the bottom left of the screen. It is icon to the left of the lit candle (Culture). Hovering on the icon will give you a breakdown of the Faith, show if it is Organized, Fervor %, who your Head of Faith is, and a brief description of the Faith. Below is a screenshot showing this.
If you decide to click on the Faith icon you will be able to see a more detailed breakdown. This breakdown will have two tabs. The first tab is Beliefs and shows Tenets, Sins, Virtues, Main Doctrines, Marriage Doctrines, Crime Doctrines,Clergy Doctrines, Special Doctrines, Head of Faith and if they are Spiritual or Temporal. The second tab shows the Holy Site locations for the Faith and the bonuses that all members of the Faith will receive if a member of the Faith holds the location.
If you hover on the number next to the Fervor at the top you will see a secondary screen pop up showing you the breakdown of Fervor and the penalties your Faith has. You can also view other Faiths’ Fervor by clicking Other Faiths right below that. This is good to take note of when declaring Holy War or if you should expect a Great Holy War any time soon.