It mostly has to do with the potential effect of believing the lie and your reasons for telling it. If you know that believing the lie results in your advantage and/or someone else's disadvantage, then it's probably bad faith. If you don't know that however, and the intent is not necessarily to mislead, that's not always bad faith. You could argue that the very act of claiming a falsehood to someone is knowingly deceptive and therefore bad faith by definition, but I don't agree that's unconditional.

For example, if I lie to protect both myself and all other parties involved, that sometimes can be in good faith! It can be bad faith if I know that it hurts them and also know a less hurtful alternative, but if I really believe the less immediately hurtful alternative will lead to a worse overall outcome then I can still be acting in good faith. It's really a lot more nuanced than "deception bad". I have to deceive myself all the time to achieve good outcomes! now I wouldn't say my treatment of myself is good faith but I try sometimes.