The correct answer is:
by communicating through word of mouth.
Explanation: Jesus and the apostle Paul actively preached the message of Christianity during their travels in the Roman world primarily through verbal communication and personal interactions. They shared their teachings and experiences with others, which helped to spread the faith. Unlike the other options, which are inaccurate (e.g., Arabic was not the common language of the Roman Empire; the Edict of Milan was issued later and did not pertain to their methods of spreading Christianity; and they did not convert people forcefully), "word of mouth" accurately reflects their approach.