First of all, if you want to give your campaign the feel of In Nomine, it might help to have a large monotheistic religion rather than the diverse pantheon of the typical D&D game. There should be more of a "black and white" morality, and have the focus on the struggle of good and evil. Also religion should play a bigger part in the campaign, rather than religion only exist to heal adventurers.
This sounds like it will be a more role-playing intensive campaign. In this case, you should think about how to handle encounters with demons. In most D&D campaigns, demons are simply high level baddies to kill and take their treasure. You might want to have encounters with demons focus not on combat, but temptation. Christianity teaches that demons tempt people to evil. The Bible talks about the devil being a tempter and deciever. So imagine a scenario where a demon tempts the party with a powerful magic item, a lot of XP, or help completing an adventure, but wants something in exchange! Also, perhaps demons could take mortal disguises so that they can decieve the party and lead them astray without the party being aware. (This is when "Detect Evil" comes in handy!)
On a side note, once back in 2e days I had a campaign with a randomn encounter chart where the party would meet the devil himself if I rolled a 00 for a random encounter. He would show up in the guise of a silly old man attempting to mislead and trick the party. Unfortunately I never rolled a 00 on a random encounter.
