The last ability of Altar of the Pantheon is a mana ability. It doesn't use the stack and can't be responded to. You'll immediately add mana and gain 1 life, if applicable.

Your devotion to two colors (for example, your devotion to red and green) is increased by one, not by two or three.

If Altar of the Pantheon loses its abilities, you'll still get a bonus to your devotion. This is because its effect is applied before any effect that would make it lose its abilities.

You gain just 1 life, no matter how many Gods, Demigods, and/or legendary enchantments you control beyond the first.