"Aliens of London" and "World War III," episodes 4 and 5 of season one form one two-part episode focusing around another grave threat to planet Earth.
The earth has been covertly invaded by a family of aliens intent on sending the world into a nuclear war, effectively wiping out the human race and turning the earth into a radioactive rock which can then be sold to scavengers from across the universe.
The special effects are perhaps less than special, and the directors seemed to love their shot of the aliens unzipping their human costume foreheads to allow the alien to emerge. The effects were actually somewhat humorous, akin to the early seasons of Buffy, but I digress.
This episode had less to do with the intersection of scifi and religion, so I will keep it short.
We learn a little more information about the Doctor. In "Aliens of London." Specifically, we learn that he is around 900 years old. How does he look so young?
We do get some interesting insights in "World War III" about the psychology of the Doctor. He is asked to promise by Jackie, Rose's mom, that Rose will be safe, a promise he is reticent to make, and in fact never does. This desire to keep Rose safe, and his apparent knowledge that he cannot guaranty her safety, seems to paralyze the Doctor and keep him from acting.
In the climactic scene, Rose, the Doctor, and Harriet Jones, MP Flydale North, are trapped in the Cabinet room at Downing Street. The Family Slitheen (the aliens) are about to gain control of nuclear weapons to destroy the earth, and the Doctor knows what he must do, but he cannot because it means putting Rose's life at risk. It is an interesting ethical moment, where the Doctor cannot bring himself to trade the life of Rose for the lives of all humans (including Rose, himself, and Harriet Jones, MP Flydale North). At this point, trapped by guilt, or at least possible future guilt, Harriet Jones, MP Flydale North must intervene, take control, and command the Doctor to execute his plan which consists of firing a missile from a British submarine with a target of Downing Street.
It is interesting that now, in two consecutive episodes (counting 4 and 5 as o episode) we have guilt as a primary motivating factor for the Doctor. In both cases, submitting to that guilt would have disastrous effects. In "The Unquiet Dead" it is the Doctor's guilt which causes him to act to bridge the Rift and he almost succeeds in condemning humanity to death, and here, in "World War III" his (future) guilt keeps him from acting, which would similarly lead to the condemnation of humanity to death.