Upon entering Oakfield, explore the area if you like; there is a woodcutting job available and a side quest involving a sculptor if you are interested.
When you are ready make your way up the path on the hill near the water wheel. Continue along the worn path, grabbing the silver key sitting in plain sight right near the path as you go. Keep going up the hill and to the left to find the Temple of Light. Enter the temple proper for a scene.
It turns out that the abbot is not too keen on trusting you with an important job without knowing if you are up to the task, so you will have to go back to town and do a few tasks in order to build up a reputation as a hero before he will trust you with his mission.
To do this, we need to return to town and pick up a quest to boost our renown.