Why do you think all the areas surrounding the mediterranean sea were christian areas by 476

Due to who was settled in the surrounding areas, Christianity was the primary religion and of course was spread.

Because there settled judaism.

Explanation:

Hebrews were the primary race from them came the rest of religions.

Christianity itself is a derivation of hebrew religions, judaism. And this cultural was settled around the mediterranean sera, which comprehend Western Asia, North Africa and Southern Europe, these was the regions of the major empires in history which maintained a close relation with jewish people.