Question

What style of art was adopted as a means of spreading Christianity during the Counter-Reformation?

Answer 1

It was Baroque.

Baroque as a style of art followed the Renaissance. It started about 1600 in Italy and quickly spread over whole Europe. It used exaggerated motion, clear details, and illusions to produce drama, tension and grandeur. All that should present the power of the Catholic Church and make people feeling small and unimportant in their relationship with God.
The baroque art and architecture was a component of the Counter-Reformation movement.