It's often stated that the first Old Believers (Starovery) started arriving in Estonia towards the end of the 17th century (see for example here). I am not sufficiently familiar with the subject to know if anabody has managed to pinpoint the exact time and place for the first wave of these immigrants, but I recently found a letter which suggests that it may have happened in late 1691 or early 1692. On 15 February 1692 Emanuel Eichler wrote from Dorpat to Governor Soop in Riga, reporting that Russians belonging to "a particular sect" had settled by the river Embach near Suspel and Tarwast as well as "on this side of Narva". Eichler writes that these Russians kept very much to themselves and baptized their own children. They would not touch food prepared by others and if they had to buy grain from local peasants they would wash and dry it once more. Some locals had already joined their congregations and Eichler feared that more would follow. How did Soop think that the matter should be handled?
I have not yet come across Soop's answer, but hopefully a copy is preserved in op. 1, vol. 46 (German letter book for 1692)
Source: LVVA, Fond 7349, op. 3, vol. 55, pp. 14-15