The source of the word "enthusiasm" comes from the Greek word enthousiasmos, which ultimately comes from the adjective entheos, “having the god within,” formed from en, “in, within,” and theos, “god.” Since its introduction in the English language in the early fifteenth century, its meaning has become disassociated with religion and god (the word god is "a god" and not "the God", because of its Greek origin) and now means great excitement for, or interest in, a subject or cause. It is this excitement, or the feeling of having a god (or the God) within oneself, which is the fuel by which success is powered.
I generally do not think the modern day definition of the word does it justice. Enthusiasm is NOT the same as just being excited. Being enthusiastic about something is very much like being inspired by a supreme being. One gets excited about going on a roller coaster. One becomes enthusiastic about building a roller coaster. Enthusiasm is excitement with inspiration, motivation, and a pinch of creativity.
Enthusiasm will empower you to do just about anything you want, but most of all it will help you do it exceptionally well. The feeling of enthusiasm has amazing effects on the physical body as well. Voice, posture, heart rate and energy, to name a few, are all positively affected when one is filled with enthusiasm. Those around you easily detect this enthusiasm. When another finds you enthusiastic about something, it is difficult for that person not to share some of your enthusiasm. This is incredibly important when selling a product, service, or idea to others. With enthusiasm, your success rate increases phenomenally.
Enthusiasm springs from desire and passion. It is difficult to become enthusiastic about something such as taking out the garbage, but certainly possible. For example, an inventor on his quest to automate the taking-out-garbage process could enthusiastically take out the garbage over and over--the source of his enthusiasm being the idea of revolutionizing the garbage industry. A great way to become enthusiastic is to visualize a time when you were enthusiastic about something. Think about what you were feeling, what you were seeing, and what you were hearing. Replay the "scene" in your head until you begin to start feeling the enthusiasm once more.
Use words, body language, and visual aids to share your ideas. Use enthusiasm to share your feelings. The ability to allow others to share your passion, even if temporarily, is priceless.
Another source elaborates a bit more.
Ralph Waldo Emerson said, “Everywhere the history of religion betrays a tendency to enthusiasm.”
These two uses of the word enthusiasm—one positive and one negative—both derive from its source in Greek. Enthusiasm first appeared in English in 1603 with the meaning “possession by a god.”
The source of the word is the Greek enthousiasmos, which ultimately comes from the adjective entheos, “having the god within,” formed from en, “in, within,” and theos, “god.”
Over time the meaning of enthusiasm became extended to “rapturous inspiration like that caused by a god” to “an overly confident or delusory belief that one is inspired by God,” to “ill-regulated religious fervor, religious extremism,” and eventually to the familiar sense “craze, excitement, strong liking for something.”
Now one can have an enthusiasm for almost anything, from water skiing to fast food, without religion entering into it at all.