Who says flat earthers are Christians or Christians are flat earthers? Millennials are the ones most likely to be flat earthers. Flat earthers didn't even exist until relatively recently.

You can call yourself by whatever names you want but this much is 100% certain:

(Philippians 2:5-11 NIV) Your attitude should be the same as that of Christ Jesus: {6} Who, being in very nature God, did not consider equality with God something to be grasped, {7} but made himself nothing, taking the very nature of a servant, being made in human likeness. {8} And being found in appearance as a man, he humbled himself and became obedient to death-- even death on a cross! {9} Therefore God exalted him to the highest place and gave him the name that is above every name, {10} that at the name of Jesus every knee should bow, in heaven and on earth and under the earth, {11} and every tongue confess that Jesus Christ is Lord, to the glory of God the Father.