Breaking your oath as a Paladin is a huge decision with huge consequences in Baldur's Gate 3, as it takes away your active subclass and replaces it with the Oathbreaker subclass. But all is not lost; the Oathbreaker class is exceptionally viable, meaning you don't necessarily have to revert to a previous save.
This combo shouldn't be possible | Baldur's Gate 3 Oathbreaker Paladin Pact weapon is insane
If you have broken your oath, look no further.
As the name suggests, the Oathbreaker Paladin is a Paladin who breaks their oath, and what this entails depends on which oath you took. When you break your oath, an Oathbreaker Paladin will appear before you, weakening your paladin powers.
He will then summon you to meet him at your camp, where he will grant you the Oathbreaker class. There is a path back to regain your oath, so don't be afraid to test yourself to see if you are an Oathbreaker Paladin.