"Christians," assuming by that you mean people who lived in the Middle Ages, did not believe that the earth was flat. I have answered multiple questions on this before:

https://www.reddit.com/r/AskHistorians/comments/3g5hpj/educated_people_in_the_middle_ages_knew_that_the/

https://www.reddit.com/r/AskHistorians/comments/3uufv2/when_did_people_understood_the_concept_of_time/

And you may also be interested in this answer from u/qed1:

https://www.reddit.com/r/AskHistorians/comments/4n981p/how_common_was_the_knowledge_that_the_earth_was/