Obama doesn't want America to give in to the "dark impulse" of prioritizing ME Christians as refugees. A nation's ability to accept refugees is finite. Given that, selectivity is perfectly rational.
When Christians are being targeted by Muslims for rape and crucifixion, when Christians, unlike Muslims, pose no threat, when the character of the host country is itself Christian, it is not a "dark impulse" to say we will bring them in.
Discrimination is not always bad. The wilful, blind failure to rationally differentiate is the dark impulse.