There are multiple ways of turning it on, But I think there's something in one of the options menus if you're not into fumbling around with the files.

The first command you will want to enter is
sv_cheats 1
This will allow you to input the next command.

Now you will want to enter
sb_all_bot_team 1
This command basically tells the game "Hey, Don't restart the map until every survivor is dead. Not just the player." Granted, The bots do asbolutely nothing without player guidance. But the important thing is making it not restart once you leave the survivor team.

Now is where we jump into the game. Enter the command
map observatory_c1
This starts up the first map of the first Campaign, "Wind of Destruction". I haven't tested this trick out on the other campaigns (And for some odd reason, I couldn't find the maps for the last 2 campaigns on the menu), But I have no reason to believe that it won't work on the others.

By now, It should be playing the opening cutscene with the Priest talking about how the 4 survivors are supposedly our only hope. But you know the old saying, "If you want something done right, You have to do it yourself."

Wait until the Priest disappears completely and his HUD icon is removed. Once this happens, You will need to open up the Developer Console and input the magic words.

jointeam 1
In Left 4 Dead (The game that Revelations 2012 is based on), There were 3 teams that could be joined with the jointeam command. Spectators(1), Survivors(2) and Infected(3). However, Don't try to join the Infected. Doing so crashed my game. The only safe way to do this is to join the Spectator team.
Note: If you did this without setting "sb_all_bot_team" to 1, The map will restart. This is not what we want to happen.

Now you're going to want to close the console and then open it up again. When you open it, Enter this.

jointeam 2 Priest
This tells the game, "Hey. When you put me on the survivor team, Would it kill you to make me take control of the Survivor named Priest?" It should also work with bots such as "Ben" and "Kelley". However, You can just start a campaign and select them without enabling cheats before you start the map.

Close the console again, Look at the bottom right icon. It should be the Priest. But there's a problem, He's kinda dead. This is going to be a huge problem, Because the Survivors can't leave the starting area and find a closet without Player guidance.

You're going to want to open the console once more and input
respawn
This command is exactly what it says it is. A way to respawn the player, Should you die.
And die is exactly what the Priest did, According to the game.

After inputing the command and closing the console, You should notice some weird stuff that you would never notice in an average playthrough.

All 4 pictures of the survivors are together on the Left of your screen, You don't have any hands to support your magical floating gun, Pressing tab doesn't show you as a player, Your icon is the Priest and the vocalizer options do nothing.
You may be wondering, "Is this it? Just playing as a fifth survivor?"
If so, Press the dance button. It should look something a bit like this.




You hold your hands in front of you forever, Your gun just sits on the ground besides your feet, You have virtually no animations and you're extremely tall.

It is at this point, You realize that you're the Priest.

And you're probably thinking, "Is there anything special that the Priest can do?"
Well, Not in particular. But there's a guide telling you how to avoid fall damage out there if you want to pretend that it's an exclusive ability. The bots sure can't do that, So it kinda is.