The worshippers knelt reverently before the holy altar to pray.

reverently 是形容词 reverent（虔诚的）的副词形式，由词根 revere（尊敬、崇敬）加上形容词后缀 -ent 构成形容词，再加上副词后缀 -ly 构成副词，表示"恭敬地、虔诚地"。