The correct answer is: Constantine declared Christianity legal, and it quickly spread throughout the land as Christians were free to worship and were given important leadership positions.
Explanation: Constantine played a pivotal role in the spread of Christianity by issuing the Edict of Milan in 313 AD, which granted religious tolerance throughout the Roman Empire and legalized Christianity. This allowed Christians to worship freely without fear of persecution, which significantly contributed to the rapid growth of the religion. Additionally, as Christianity gained legality and prominence, many Christians were appointed to important leadership positions within the empire, further facilitating its spread and integration into society. The other options presented do not accurately reflect the historical context or outcomes related to Constantine's actions and the spread of Christianity.