If Defender of the Faith doesn't get taken too early, the player will at least be strong enough that it's more of a nuisance rather than gamebreaking.As one of you already suggested, lock the option becoming DotF behind a minimum of prestige let's say 50.
That would be simple and effective. It prevents awful starts, but doesn't prevent AI from picking it later in the game.
If you still struggle with DotF later, it's just your fault either swapping religion too soon or not waiting for a good moment to attack.
I really think it should only come active during the Reformation and apply to Catholics/Protestants, or it should only count if the defending nation is being attacked by Heathens.
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