So True Believer Feat (Complete Divine, MiC) is a feat that gives you once a day (chosen by you) a +2 Untyped Bonus to any of your saves, but once a day. You can also use a Divine Relic without giving up a divine spell slot.

Ask yourself if you need more than once a day use, how much do you need the additional flexibility of having it up all the time, multiple times per day, and you do not have to choose is this the best time to deploy said feat? Is once a day good enough especially since you can now use a divine relic which may have a useful benefit.

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Another way to look at it is this feat is like a Cloak of Resistance +2 so 4k if you bought it at a store. Except it stacks so if you already have a cloak of resistance +1 (1k) upgrading it to a +3 is 8k of benefit, or upgrading it from a +3 (9k) to a +5 is a 16k benefit.

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Another way to look at it is to compare it to Ability Focus which is also a +2 DC benefit, while the Improved Saves is a -2 DC benefit. Now there are trade offs here for offensive is usually better than being defensive but here is the thing about DCs, it is a first past the post system where you just need to hit your target and any extra is wasted besides altering the probability of effect. You either made your save or you did not, you either took out an enemy from the fight or you didn't and running up the score provides little benefit.