I see a lot or conjecture on this thread. The only valid statement I found is from Adam Fletcher, global community development director, that answers:
“You can play without but things like chat, partying and world PvP will require.”
Source:
apparently not, Adam Fletcher says the opposite 
I edited the post to correct it.
psn says online play required for d4, unlike d2r where it states optional.
From the Xbox website when looking at Diablo “Online multiplayer on console requires Xbox Game Pass Ultimate or Xbox Live Gold (subscription sold separately)”.
Which just feels weird needing a subscription to chat, party up, and engage in PvP.
Does this mean I can just fly through PvP areas unbothered? 
And one will just be mute and can never party up despite not needing a subscription service on PC
Guess we get a monk class on launch!
Hi, so just to confirm the game can be played offgrid?
Like take you PS and go RV go offgrid and can still play? Just like Diablo 3 was?
No, it requires an internet connection to play. The game is always online. No offline mode.