What religion did the early European explorers how to spread

This answer really depends on which country and how early you are looking at, in terms of which era of explorer.

The answer, broadly, is Christianity.

Depending on the explorer and the country they represent, though, the answer could be Roman Catholicism or Protestantism.

The correct answer is - Christianity.

The early European explorers, apart from conquering new lands, spreading their culture, technology, they also were spreading their religion. The religion that they were spreading was the Christianity, depending on the country, it was either the Roman Catholicism, or the Protestantism.

They managed to spread out their religion on practically all continents in the world, apart from Antarctica, and it became the dominant religion in the three newly discovered continents of North America, South America, and Australia.