Esther 8:17 (ESV)

17 And in every province and in every city, wherever the king’s command and his edict reached, there was gladness and joy among the Jews, a feast and a holiday. And many from the peoples of the country declared themselves Jews, for fear of the Jews had fallen on them.

As far as the edict travelled, it gladdened the Jews and made a deep impression on the rest. Was it the power of the Jews that caused the other peoples to be seized with fear? This is highly unlikely; the Jews were a small minority and lived in scattered groups. No, this fear was that which had formerly seized the Egyptians and Canaanites. It was a fear that came from the God of the Jews (Exodus 23:27; Deuteronomy 2:25; Deuteronomy 11:25; Joshua 2:9).

The dramatic deliverance of the Jews served as testimony to the power of their God. Many people now publicly declared themselves Jews. Most likely, this would involve circumcision for the men, after which they would begin worshipping in a Jewish assembly.