The European power that focused their colonization efforts on the conversion of Native Americans to Catholicism was the French.
Explanation:
Along with other operations, the French were also very much interested in the dissemination of Christianity in the regions newly colonized by them.
The spread of Christianity was one of the major points on their agenda for a long period of time in the history of colonization. It was for this reason that they undertook campaigns to convert the native Americans to Catholicism.