To unlock the Divine Intervention trophy in Assassin's Creed Odyssey, you must first make sure you have the latest game update installed -- this content was added in a free patch with 1.07. You also need to have cleared Chapter 5 of the story.
You need to complete a series of quests in the Korinthia region. The missions themselves are very easy assuming you are levelled up appropriately.
The nearest fast travel point is the Temple of Apollo in Korinth. Head to the quest marker and speak to The Mysterious Woman.
Test of Judgement
Head to the graveyard and after investigating the area you will be confronted by a man. I believe there is no way to avoid attacking and killing him. Go back to the Mysterious Woman and choose Apollo or Poseidon (this will determine whether you receive an Epic Sword or Spear at the end of the quest arc, and also which quests you receive next).
Test of Faith (Apollo strand)
Head to the marker to talk to the village elder, then head out to Aegina Island across the water. Talk to the lion man. Either kill him, or complete a small fetch quest for him which will involve killing some lions at an abandoned camp. Talk to the lion man, then the village elder, then the mysterious woman to complete the quest.
Test of Adaptability
Head to the marker to talk to the hermit. Then head into the Ariabignes Shipwreck to get the key. Talk to the mysterious woman to complete the quest.
If you picked Poseidon, you will receive a slightly different set of two quests from the above. The steps are similar.
Test of Courage
Ever wonder what was going on at The Depth of the Forge? Now you can find out. Dive in to the Forge from the view point on the barren volcano. Platform across the lava and head through the previous inaccessible door. Don't forget to pick up your Spear or Sword, which will either be to the left or right depending on which god you chose earlier. Retrieve the helmet and return to the Mysterious Woman to finally finish this quest arc (you'll get the helmet back as a reward too).
The trophy should unlock shortly after you finish this quest.