Devotion

Devotion is a mechanic introduced in Theros, where it was connected to the god theme of the set. It appears as "devotion to [color]" or "devotion to [color 1] and [color 2]", where color is any of white, blue, black, red, or green.

Description
Devotion to a color is defined as a numerical value that a player has, equal to the number of mana symbols of the color(s) in the mana costs of permanents they control. There are cards that count devotion to each of the five colors, and they have many different effects based on that devotion.

Devotion is a mechanically tweaked version of chroma with a new name and flavor.

The mechanic appeared throughout the Theros block, and made a reappearance in Theros Beyond Death.

Rules
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Rulings

 * Numeric mana symbols (,, and so on) in mana costs of permanents you control don't count toward your devotion to any color.
 * Devotion counts only mana symbols in your permanents' mana costs—the cost in the upper right corner of the card.
 * Mana symbols in text boxes of permanents you control, such as activation costs or the big mana symbols on basic lands, do not count toward your devotion to any color.
 * Only colored mana symbols count; it doesn't matter how much a permanent costs to cast, just how much colored mana.
 * If an activated ability or triggered ability has an effect that depends on your devotion to a color, you count the number of mana symbols of that color among the mana costs of permanents you control as the ability resolves. The permanent with that ability will be counted if it's still on the battlefield at that time.
 * Hybrid mana symbols, monocolored hybrid mana symbols, and Phyrexian mana symbols do count toward your devotion to their colors. A hybrid mana symbol adds one to your devotion to each of its colors and one to its combination of colors; it does not add two devotion to its combination of colors.
 * Mana symbols on permanents with devoid still count; devotion just counts mana symbols, not the colors of the permanents themselves.

Examples

 * A permanent that costs adds one to your devotion to black.
 * A permanent that costs adds three to your devotion to red.
 * A permanent that costs adds one to your devotion to green, one to your devotion to white, and two to your devotion to green and white.
 * A permanent that costs adds two to your devotion to red, two to your devotion to green, and two (not four) to your devotion to red and green.

Trivia

 * Bloodthirsty Ogre and Pious Kitsune from Champions of Kamigawa use devotion counters, which mechanically share no similarities with the devotion mechanic.
 * Increasing Devotion from Dark Ascension was conceived before the Theros block and shares no mechanical similarities with the devotion mechanic.
 * Devotion to colorless is supported by the rules, but is narrow and confusing.