You might have trouble with a bit of his theology (he is a Christian)
Trouble? As in he's an Arminian Christian or what?
No, I’m not saying that he’s Arminian, though he might be, but he’s an authentic expert on celtic lore and the transition of pre-Christian celtic culture into Christian celtic. One of his series places the repository of real Christianity in Scotland, so for all I know he has a strong appreciation for Calvin and kirks.
Best to read his “Byzantium”.
Also, best to keep in mind that it’s not historical fiction but is more like historical fantasy.