The Polish publication Biala
Podlaska – Brzesc nieodkryty wschod (PDF, in Polish, here) on page 41, tells us why the Jews came to the Grand Duchy of Lithuania, and confirms Zonnenberg:
The Jews appeared on the territory of the
Grand Duchy of Lithuania at the turn
of 14th-15th centuries. Seeking refuge from religious persecutions and pogroms in Western
Europe, they came to the religiously tolerant GDL to settle in towns and townlets. By
a special decree the Grand Duke Witold allowed the Jews of Brest
to build synagogues in the town.
In the 16th-17th centuries the Brest Jewish community was very powerful
and the Brest
synagogue was one of the most famous in
Europe.