1) The Priest is a strong character who is being attacked by false witness.
2) The aliens have id'd the soul as the source of humaness and are attemting to find out where, how, why, what it is.
I think the whole soul question could be mined for interesting plot developments.
I remember when Tbs or TNT were beginning to show reruns of Angel (after it had been cancelled by its network). The promo spoke of a "vampire with a conscience". The promo could not bring itself to use the word "soul", yet the show used it constantly.