Lettonia was evangelized around 1200, and became known as the "Land of Mary," a title confirmed by Pope Innocent III very soon after.
The Virgin was in a way the launching base of its Christianization. All solemn promises were pronounced in the Cathedral of Riga, in front of the altar of the Virgin and before an image which unfortunately was thrown into the river during the Protestant Reformation.
The most important Marian shrine is in Aglona.
Source: Madre della Chiesa dei Cinque continenti, by Attilio GALLI, Ed. Segno, Udine, 1997.