The fellow wrote that Muslims were lost and had not found God, something like that. There is every suggestion that this might pose a problem for his function as a civil servant. Now, the right answer would have been 'yes, I can separate the two, I will look after the interests of all Americans regardless of faith'. He couldn't do it. Every sentence was 'because I am a Christian ... such and such'. I'm sure his past experience has been that that kind of grandstanding goes over very well and he couldn't imagine anyone calling him out on it.
Everybody has said or written something at some point that I'm sure they regret, that's not the point. He wouldn't, or perhaps didn't know how, to address the issue.