If I didn't activate my find my iPhone is it possible for the thief to reset factory settings? I just got my iPhone stolen. I quickly turn on find my iPhone. I wonder if before I did that the thief have reset factory setting first. Is it possible?

It seems that the mere act of setting my iPhone on password should make the iPhone unusable even without find my iPhone feature. Am I correct here?
If you haven't enabled Find My iPhone, then you haven't enabled Activation Lock, which means the device can be reset simply by placing it in Recovery Mode and clicking Restore in iTunes. Setting a passcode protects the information on the device but won't prevent against restoring the OS.