What is the default Faith Acceptance game rule in CK3?

Default: A Faith's attitude towards other Faiths is dictated by their doctrines. (patch 1.20.1.2)

Does changing the Faith Acceptance rule disable achievements?

Yes for “Full”: the file gives it the flag blocks_achievements. The other settings keep achievements on.

Game rule · Game Modes · Faith

Faith Acceptance

Default
Full ✕

Faith Acceptance is a game rule in Crusader Kings III (Game Modes and Faith) with 2 settings: Default (the default) and Full. “Full” disables achievements. Patch 1.20.1.2, from game/common/game_rules/00_game_rules.txt.

This section as JSON: /raw/game-rules.json

Quick answers (2)

What is the default Faith Acceptance game rule in CK3?: Default: A Faith's attitude towards other Faiths is dictated by their doctrines. (patch 1.20.1.2)
Does changing the Faith Acceptance rule disable achievements?: Yes for “Full”: the file gives it the flag blocks_achievements. The other settings keep achievements on.

2 settings

Default
Default
A Faith's attitude towards other Faiths is dictated by their doctrines.
default_faith_acceptance
Full
Disables achievements
All Faiths have full acceptance towards other Faiths. There are no Opinion penalties for having a different Faith.
The script checks it once, in 1 place
On actions (the game's own hooks) 1
- on_game_start_after_lobby
Every has_game_rule = full_faith_acceptance in game/common and game/events, by the top-level object it sits in.
full_faith_acceptance

Source: game/common/game_rules/00_game_rules.txt line 1033 (key faith_acceptance), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.