When the word of the Jews' defense spread throughout the kingdom, the Megilla tells us (8:17) that many non-Jews converted to Judaism. The present tense verb, "mityahadim" - literally, were converting - is used instead of the seemingly more appropriate past tense "hithayadu" - converted. Why is this so?
Perhaps the Megilla is hinting that their conversions were insincere and therefore unacceptable. That same verse gives the reason for their conversion, "because the fear of the Jews had fallen upon them." Fear is not a legitimate motivation for conversion....
In the Purim miracle the Hand of G-d was hidden. This is why the Name of G-d does not appear in the Megilla. It appeared to the non-Jewish world that the Jews had gained power through this-worldly forces. Therefore, they reasoned, it was wise to be on the Jews' good side. What better way than converting? However, these converts, motivated by fear, would not be accepted. Therefore the Megilla says that they were converting, "mityahadim," but does not say that they actually become converts -"hityahadu".
Read more Divrei Torah about the megilla at: