There are many synonyms for the phrase "extremely enthusiastic," each communicating different shades of intensity and passion. Some options include fervent, passionate, ardent, zealous, eager, avid, spirited, enthusiastic, intense, and fiery. Each of these synonyms suggests a deep and abiding commitment to a particular cause, idea, or activity, and can be used to describe individuals, groups, or even entire movements that embody this kind of energy and passion. Whether you're talking about a sports team, a political movement, or a creative pursuit, these synonyms offer a rich and varied vocabulary for capturing the power of enthusiasm in all its forms.