Overview

General

PvP will absolutely be part of Pax Dei and will feature various ways to "opt-in". One pretty much confirmed way will be "zone-based".
Here are some important statements form the developer in that regard:

PvP is opt in insofar as it will be clear when you enter that mode or when you enter a zone that has dangers of PvP. If you do not want to engage in PvP then there is plenty of PvE content in the game and you don’t have to engage with the game that way at all. The same is true for PvE. If you prefer to just be a master crafter in your village and never venture out, then that is possible too and will be just as important in fact as other activities. There are always tradeoffs naturally. As mentioned in another dev-question, many things are gated via skills. Aspects of combat as well as crafting an others. This results in many ways of requiring to work together. You might need to go into a high level PvP zone to mine a certain ore, and bringing a bunch of PvP players along to protect you is surely desirable in this case, especially if you plan not to be part of it. However this isn’t a must, opt-in is the keyword for every activity. The choice is yours and it will be clear when you play what dangers you are exposing yourself to.

While PvP will be opt-in, we really want to make something that has high stakes, is in-world and has good reasons for why you want to PvP. If this pans out the people really into PvP (and we have some Very Serious PvPers in the dev team) will really like this, the people who want to test waters with PvP will be able to and there's a good reason for PvPers and PvErs to work together.

PvP will definitely NOT be forced, and it is up to you to engage in it. If you do engage in it, there will be benefits for everyone. We want PvP to be something that PvPers do enjoy a lot and can find meaning in as well be something that PvEers can 100% ignore and still get the best stuff in the game.