Though I can not be certain, I assume that demons believe that God exists as the Trinity, and that God is holy, righteous, and good, and that Jesus was God
incarnate, born of a virgin, lived a sinless life,
died on the
cross, and rose again from the dead.
He would not say, with Anselm, that if God wished to forgive men's sins He was bound to become
incarnate and
die on the
cross, nor did he hold, with Aquinas, that the incarnation and the crucifixion were the wisest way to achieve man's salvation.