I seriously doubt that the main quest is not in your quest log if you have gotten this far. Don't click on the spoilers if you want to figure out what to do yourself.

Going to the source temple is the main quest


If you want to know when and where you can use the soul forge:
After you have completed the source temple and returned to the homestead you can use a new portal to head to the final battle. Leandra will confront your party and you can choose to use the soul forge by selecting the correct dialogue option.