The church gain power in the middle ages because

Roman Catholic churches emerged to be the one aspect which unified Europe in the middle ages. Church is the center of activity for all Christians. They were much devoted to Christ.

Many feudal lords and continental people contributed 10% of their earnings to the church and the church was exempted from paying taxes. Gothic architecture and Frescoes paintings in cathedrals were spreading the beliefs in Christianity.

The sermons continued in Latin. Popes exercised much power on the kings of Europe. Anyone who opposed the church was banished and not permitted to participate in sacraments of the church.

Most people in that age believed strongly in God. The Catholic Church was the center of that belief, people thought that they could get to go to heaven only through the catholic church (god was the center of the universe)