The pope’s letter in the Italian newspaper La Repubblica, from which the quote above is taken, is getting a lot of attention.  I wish he would have expounded on the matter more, but as far as official Church teaching on salvation, atheism, and conscience (which, I feel confident the pope adheres to) note these quotes from the Catechism of the Catholic Church:

CCC 848 “Although in ways known to himself God can lead those who, through no fault of their own, are ignorant of the Gospel, to that faith without which it is impossible to please him, the Church still has the obligation and also the sacred right to evangelize all men.” (from the Vatican II document Ad Gentes, 7)

CCC 2123 “Many . . . of our contemporaries either do not at all perceive, or explicitly reject, this intimate and vital bond of man to God. Atheism must therefore be regarded as one of the most serious problems of our time.” (from the Vatican II document Gaudium et Spes, 19 § 1)

Also see the CCC on conscience: paragraphs 1776-1802.