Well, I've never encountered an Apostle, but I'm told that it's a super secret boss and very few people know how to meet it. I don't know the exact requirements, but I'll share the rumours:
You need to wield the Dragonslayer, (I've never figured out how to do that) wear the Berserker Armour, (I think you can only do that if you are wielding Dragonslayer) and then get a full win by nuking the Mastermind while invulnerable. I think that you can also meet it in the Angel of 100 challenge by getting to the last level with Dragonslayer and Berserker Armour. I'm told that the Apostle is a kind of super Arch-Vile that does crazy amounts of damage and has a ridiculous number of hitpoints.
That's what I've heard. I don't know any more details.