Well, what actually happened seems a little more complicated, according to the document wandeljw linked to.
Walden worked for a company that sells counselling services to the Center of Disease Control, a US government agency. She referred her prospective client to a colleague within her company. According to the client, this colleague did a superb counselling job. Afterwards, the client filed a formal complaint against Walden, the CDC asked Walden's employer to fire her, and her employer complied.
To me, these facts present two issues:
1) Walden's prospective client was satisfied with the counselling she received from Walden's colleague. So the CDC got what it paid for from Walden's employer. Walden caused the CDC no harm by acting on her religious convictions. Given that, what legitimate reason did the CDC have to request Walden's firing? How is this not retaliation against an inconvenient religious stand Walden took?
2) It isn't clear that Walden's actions caused harm to her employer. They didn't investigate whether she could have taken other clients instead. Given that there is no clear showing of harm, what legitimate reason did Walden's employer have for firing her? I realize that this is America. Walden's employment might have been "at will", so her employer might have been able to fire her for any reason, or for no reason at all. Still, it isn't clear from the documents that Walden's behavior affected her employer, so it isn't clear that the employer had a reason to fire her.
It'll be interesting to read the defendants' side of the story.