This sort of grasping at straws to try to make every single act of violence or terrorism committed by a white guy "Christian terrorism" really doesn't help and distracts from the real horrors. We don't even know the perpetrator's religious beliefs. No one would call Popular Front for the Liberation of Palestine (whose founder isn't even a Muslim) an Islamic terrorist group even though they have the same goal as Hamas, because their attacks aren't religious motivated. Neither was this guy's, even if he was loosely connected to the KKK.
It's kind of amazing how people also think the Oklahoma City bombing was an act of religious terrorism when McVeigh wasn't religious in any way...I've even heard people call Dylann Roof a Christian terrorist even though he actually attacked a Christian place of worship. It's amazing.
The alt-right loves
Christiandom, but actively and publicly rejects Christianity, often preferring edgier belief systems like Paganism.