Enthusiasm literally means being filled by or possessed by a god. (EN= in; THUS=theos)
Enthusiastic new believers lead more people to commitment to Jesus than seasoned evangelists do. Enthusiastic salesmen sell more products than salesmen who know more about the product and the customers by better education in those areas. Puritans made the term enthusiastic an insult because they didn’t believe anyone could receive revelation, so they mocked the idea that someone was filled.
Enthusiasm is what Paul was describing in Philippians 2:12-13:
So then, my dear friends, just as you have always obeyed, not only in my presence, but now even more in my absence, work out your own salvation with fear and trembling. For it is God who is working in you, enabling you both to will and to act for His good purpose.