What natural features served as the northern border for Christian expansion

Ocean and Rivers

Explanation:

Ocean and Rivers play a significant role in serving as the borders for early Christian expansion (Romans). It protects them from rivalries or enemies attacking. AS Romans were expanding their empire across Europe, they use the natural features to serve as a natural defence for outside force who might attack them. Even if any enemy came to attack, the Romans could observe them crossing ocean and rivers before the attack and prepare it on time, which would ultimately be victorious for them.