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It may be helpful if her mother prepares her for your disclosure. She is responsible for withholding the information from both of you for whatever reason. You will not be turning Faith against her mother because you will not be saying anything negative or accusatory. You can let her know that just like her, you did not know you were her Dad.
If possible, the three of you can have an outing and her Mom can prepare the ground by talking about parenthood and how life had been growing up not knowing your father. Then, she can help you with your disclosure.
Children feed off of adult's vibes. If there is tension or animosity, she will pick up o that. On the other hand, if you and her mother are honest and help her get used to the idea, she may actually be relieved to have learned that you're her Dad. Try to learn about Faith and with time, the two of you can establish a relationship.