Can you describe for me the primary characteristics and symptoms that distinguish religious enthusiasm from religious fanaticism?

Let’s begin with a general distinction between enthusiasm (intense and eager enjoyment, interest, or approval) and fanaticism (excessive, uncritical zeal or enthusiasm), and then apply it to a religious context. A religious fanatic, at some level, is ignorant or intolerant of other views. Among devotees, for example, a fanatic might not recognize the diversity of bona fide views, or the good qualities of other religions. S/he might make unreasonable demands on fellow religionists, or those outside the fold. S/he might also confuse optional details with basic requirements, etc.

How can I maintain enthusiasm in Krishna consciousness without being infected by religious fanaticism?

HDG: Our enthusiasm should be based on the glory of Krishna. Since Krishna dwells within everyone and all things, and everyone is somewhere along the single path leading to Krishna, we can be most enthusiastic about Krishna without being negative or offensive toward other souls, both those within ISKCON and those outside it.

And how should a Krishna conscious person respond to religious fanaticism?

HDG: They should not respond with counter-fanaticism. Krishna says in the Gita 9.9 that He Himself remains above [the fray], ud-asina. And at 12.16, he says that a devotee who is also ud-asina is dear to Him. At 14.23, Krishna says that being ud-asina is a symptom of transcendence. Of course a devotee does eagerly try to serve Srila Prabhupada’s mission, but we should not unnecessarily get involved in quarrels or conflicts. This however does not preclude us from speaking the truth as we see it. But, as Krishna states at 17.15, austerity of speech is to speak that which does not agitate, and which is true, pleasing and beneficial. Not always easy!