The "old-fashioned" definition of the word saint would be "a person of extraordinary holiness". The "modern sense" of the word saint in 1939 would most likely be defined as "a nice person". Auden uses this contrast to further neutralize the unknown citizen by letting the reader know that the word saint has changed meaning. This change has shifted from a righteous identity to an ordinary word. Thus, the unknown citizen becomes an ordinary man.
Etymology definition of "saint" link.
Etymology definition of "saint" link.