Laughs in Justinian.
It was rewriting history and being too involved in church disputes that caused the decline. Banning all other religions except Judaism. A lot of smart pagans fled to Persia and caused a brain drain as well. Then the huge Byzantine and Sassanid wars happened while they tried to ban Judaism too late. Then the Arab conquests happened and were successful because religous minorities were able to be a spy network.

Not so much Christianity itself. But Christian policies that were stupid. If they kept old laws in place regarding religion with a change in the state religion like Constantine did Romans might still be around today.