My understanding is that you have to be an ordained priest/minister in your faith in order to pursue a career in the Military as a Chaplain. As a Chaplain, you minister to all members of the military - regardless of their religious beliefs. There are also other options - you may also become a Chaplain’s Assistant, which means you would travel with the Chaplain on missions - your chief duties would be to provide security for the Chaplain, and of course, you would be responsible for other administrative duties as well.
I hope that helps with your question.