How was roman emperor constantine 1's adoption of christianity similar to charle's martel's victory in the battle of tours? a- both events made christianity the official religion of a nation b-both events legalized christianity in areas where it had been illegal c-both events were examples of christianity victory over islamic forces d-both events were examples of christianity's unifying effects in europe

The answer is D. Both events were examples of Christianity's unifying effects in Europe. 
Both emperor Constantine 1 and Charle Martel used the teachings that written on the bible as the foundation of the law in their kingdom.
When they conquered a certain region, they will impose their religion-based law to the people, making a lot of regions in Europe adopt a similar law even though they're controlled by different kingdoms.