I once blogged about the Emerald Tablet here. So, what IS the "One Thing?"
According to physicist Tom Campbell, it is one great consciousness. (Emerson called it the Oversoul; others call it God.) Reducing your own entropy (the clutter in your ego-brain via meditation) and raising your own consciousness will take you to a higher level of perception in which you experience the One Thing.
It's tough to explain, but as an applied physicist who has conducted numerous experiments and has himself had out-of-body experiences and traveled across time and across universes, Campbell explains what he means in the video below. It's 90 minutes long, but well worth your time.