Your employer can almost certaiinly fire you. An employer may not fire you simply for  being religious. And an employer must make reasonable accomodations to religous practices, such as letting an observant Jew leave early on Friday and make up the time at some other point, or allowing a Muslim woman to wear a head dress, or allowing a Christian to not work on Sunday morning.However, only "reasonable" accomodations for mandated religious practices must be allowed. A mission trip may be encouraged by your church, but is almost certainly not a mandated practice, such as observing the sabbath--there are people who will never go on a mission trip. Therefore, it is not something which must be accomodated. Second, taking 2 weeks off from work is not reasonable--it can be very disruptive to your employer and unfair to coworkers who have to cover for you.If you have sufficient vacation time, you should be able to use it for thhis purpose, as you could for any purpose.