The Rain Miracle
The time was winter c. 168-174 AD, when Roman Emperor Marcus Aurelius led his army against pagan enemy tribes in the northern part of the empire. During the fighting the army of Rome, exhausted and thirsty, was surrounded by the far more numerous barbarian enemy Quadi, and cut off from all avenues of water. It was written by Roman Historian Cassius Dio that the Christians among the Roman troops started praying to God for salvation, not only for themselves but also for the Romans of the 12th Legion as well as their emperor.
The Thundering Legion
The English translation of the name given to the troops by Marcus Aurelius, Legio fulminata, means Thundering Legion. Cassius Dio wrote that the sudden violent thunder and lightening storm so frightened the enemy that they ran off and the 12th legion was saved by the downpour of life saving rain that came in answer to the praying Christians.
From all of the writings of the incident, there seems no doubt that there truly was an unusually brutal storm with severe winds, slashing rain, and firery thunderbolts that drove away the enemy, but there have been pros and cons over the ensuing centuries about the reason for the deliverance of Marcus and his troops.
Early Christians were persecuted harshly by Roman Emperors, that seems an inescapable fact from all that has been written about the mean atrocities such as the feeding of Christians to lions in the Colosseum under Nero and others. By the time of Marcus Aurelius, however, Christians were in the army of Rome, attested by the cleric and historian Eusebius. If this were the case, why would the people of the Prince of Peace not include Marcus and the Romans in their prayers?
Another point that has been debated is the letter that Marcus was said to have sent to the Senate praising the Christians for their prayers. He also reportedly called the sudden storm a miracle.
Sources: same as above.