I don't know anything about Capt. Paul Fields or his record as a police officer. I'm guessing he is the sort of man who started off as a patrol man and worked his way into an administrative job over the course of his career.
There are a couple of things we know about him. The first is in the course of his official duties as a police officer, when criminals have threated to harm a mosque and those attending it, he has provided them with proper police protection. That is his job. He is a police officer doing what is expected of him, serving and protecting the rights of the citizens in his jurisdiction.
The second thing we know about Capt. Fields is that he is a Christian that doesn't want to spend his time being proselytized by Muslims. Both of these things seem very normal to me. After all there are any number of people that I serve in the course of my job. I try to do the best I can for these folks and meet their needs and exceed their expectations. This is what is known as doing your job. On the other hand, if they show up on my front porch on my day off wanting we to join their religious group, I'm going to politely ask them to leave. I'm not obligated to give them one bit of my time or attention beyond what is needed to serve them at work.
What happened was Paul Fields was directed to attend, and get other officers to go to a "law enforcement appreciation day" at the local mosque. At first it was voluntary. Nobody wanted to go. I don't know why the cops under Capt. Fields didn't want to go. Maybe it was a day off. Maybe they had to work but had more important things to attend to. Maybe Muslims aren't known for having good BBQ or donuts. Maybe none of them wanted to get their picture in the paper as part of a photo op designed to promote Islam. Incidentally that is exactly what happened to some cops that showed up.
Capt. Field's employer, the city of Tulsa OK decided that Capt. Fields needed to be ordered to attend. When he refused, citing his deeply held religious beliefs, he was punished with a reduction in grade and a two week suspension. Paul Fields then sued his employer on grounds of religious discrimination.
The courts have all sided with the City of Tulsa.
In the appellate court decision it is pretty clear that the City sends officers to a number of community activities about 10% of which occur in religious facilities. In the cases of the other religious groups, it has always been a voluntary participation. Capt. Fields knew this. He even stated he had no problem with his men going on that basis. He simply refused to make it mandatory, or go himself.
Why is it voluntary for cops to go to an activity at say a Presbyterian church but its a requirement for cops to go to one at a mosque?
Would a Muslim cop be forced to attend a Jewish Synagogue, or observe an Easter Mass?
I think, at least maybe, one the reasons non-Christians don't give much consideration to the principle of "deeply held religious beliefs" of Christians is that the Christian community doesn't do a very good job of demonstrating that they have them.