[WOSG]

"Aristotle made it clear that there are undemonstrable first principles, a feature common to both philosophy and religion."

hmmm, did Aristotle say that? Did he say there were first principles? Got a quote? Did Aquinas borrow the concept from him then?

[cornelis]

It's very possible Aquinas borrows this. Aquinas wrote a quite few extensive commentaries on Aristotle's works. I'm getting it directly from the Nicomachean Ethics book 6.