The prefect of the Vatican's Congregation for Eastern offered prayers for those affected by the crisis in Iraq that has forced thousands to flee their homes.
Militant group ISIS has given them an ultimatum to convert to Islam, pay a tax or be killed.
Cardinal Leonardo Sandri also spoke about Christians in Syria, Egypt and Palestine during a mass in San Diego.
He said: "no religion can accept to kill God's children in the name of the same God."
It's thought there are now just ten Christian families left in the Iraqi city of Mosul.
At the weekend thousands of people marched in central London calling on the UK government to do more to help.
On Monday France offered asylum to Christians fleeing ISIS.