Why was Holy Roman Empire powerful?

It was virtually useless, except for ceremony and light taxation.   Vienna even had to be saved from Muslim conquest 3 times, the last Sept. 11,1693 by the Polish-Lithuanian army.

Charlemagne has been called the "Father of Europe" (Pater Europae), as he united most of Western Europe for the first time since the classical era of the Roman Empire and united parts of Europe that had never been under Frankish or Roman rule. His rule spurred the Carolingian Renaissance, a period of energetic cultural and intellectual activity within the Western Church.

They carried the biggest stick with which to control others it you're talking about the roman government.  If you talking about the church, people were afraid of losing their souls.  Also, when the Roman Emporor Constantine became a christian, he made Christianity the official church of rome.  This spread Christianity throughout the roman empire and gave tremendous power to the church.