I can see that, but it looks more like a castle being defended to me. We don’t really know what medieval mage created this sigil, but he wasn’t a druid I would put money on that.
“Knower of the truth” yes. 
The Anglosaxons also used the same word for tree and truth: treow.
Wouldn’t it be nice if we had Druid Kings again? King Arthur isn’t going to rise from his sleep though, I fear. Stick with Belial at least he’s around.
from Celtic compound *dru-wid- “strong seer,” from Old Celtic *derwos “true” (from PIE root *deru- “tree,” especially oak) + *wid- “to know” (from PIE root *weid- “to see”). Hence, literally, perhaps, “they who know the oak” …Etymology online
All trees attract lighting. All trees are sacred to the Druids. But all you need to get struck by lighting is for the leader from you to connect with one of the leaders coming down from the clouds first.
Wait I lost you. What have snakes got to do with Belial? Cernunnos is more related to Pan and through Pan to Azazel I feel.
But here we are playing the matchy matchy game, which I personally feel is unecessary, as has been discussed on BALG before.
It’s ok to commune with both Belial and Cernunnos… and Pan and attain their gifts as individuals. What’s the need being met in smushing them together? You just lost all the nuance?
I mean, he comes from a part of the world here Oaks didn’t even exist. 
You know, if you want an Oak, really… and speaking as a student Druid myself (Order of Bards Ovates and Druids, I’m not at all done with the Bardic grade) then… why not go out for a walk and talk to an actual Oak?
Tree divination, as in, using the Ogam, is a thing.
At that point, you could compare the lived experience of the energy of Oak compared to the Energy of Belial, rather than letting your conscious mind run away with the story and build castles in the sky.
This is all so intellectual and lovely and all, but Druidry and magick are practices, what does it mean if you are not practicing druidry on its own merits, or speaking to Belail on his own merits?