Terveisiä Turusta! (Finnish) = Hälsningar från Åbo! (Swedish) = Greetings from Turku!
(Thank you Tuomo)
The postcard shows the interior view of the cathedral. The oldest parts of the church date back to the 13th century. Nowadays, it's a Lutheran church but originally (before reformation in the 16th century) it was catholic.
It's dedicated to Virgin Mary and St. Henrik, the missionary martyr bishop (12th century).