Someone can be any of the following:
- practicing Christian
- devout Muslim
- practicing, devout Jew
- non-practicing Christian
- (non-devout is not usually used, and usually only people practicing a religion I would describe as devout)
"Practicing" refers to doing the actions associated with, such as praying, going to places of worship, participating on holy holidays.
a "non-practicing Christian" is someone who mostly identifies as Christian (might believe in the Christian god, went to church as a kid but doesn't go as an adult) but doesn't go to church regularly or only on Christmas and Easter. They also probably don't spend too much time reading the Bible, saying Grace before eating, or regularly formally praying.
"devout" refers to someones conviction, and means that they deeply believe and adhere to their religious belief and specific religious teachings. I would argue with @Michael Harvey that I would not use "devout" for a non-practicing religious person who is has a very high conviction to their religion. Obvious exception if there are obstacles to practicing like being held in a prison and not being allowed to do religious acts.