Here is how I did it easily:
1. Make a Spiritualistic Empire
2. Start with Scion Origin
3. Start on a Big Galaxy (not sure if matters)
4. Set Habitability on 5 and start the game
5. Scion Fallen Empire should be Holy Guardians (I tried several times and it was always Holy Guardians for me)
6. Your Scion will contact you in the first month or so and will be revealed partually on the map.
7. Go survey systems around the Scion empire for the 4 Holy Worlds - if there are more than 4 gaia worlds around the Fallen Empire check the planetary expansion tab, the holy worlds have a special icon.
And from here on farm enough unity for outposts and build 4 colony ships, my scion didn't attack me or anything when I colonized the planets.