Islamic studies researcher, Antony McRoy, also called upon congregations to be "defiant" but "not give way to any kind of fear" as they celebrate Jesus' birth.
Speaking on Premier's News Hour two days after twelve people died when a lorry ploughed into crowds at a Christmas market by the city's Kaiser Wilhelm Memorial Church, he said: "We must be vigilant and we must be realistic.
"We can't just have stewards on the door to welcome people, we must - all of us - be vigilant and look for anyone who might be suspicious."
A Europe-wide manhunt has been launched by German police for a man with links to Islamic extremists who was identified as a suspect in Monday evening's attack.
Authorities has said the suspect, a Tunisian in his early 20s, should be considered armed and dangerous.
Meanwhile, Berlin mayor Michael Mueller told ZDF television shoppers in Berlin had no need to fear, adding: "The police presence has been significantly heightened ...and of course other measures taken to find the perpetrator quickly."
Anthony McRoy said the focus of any terror threat after Christmas Day would move away from churches to "economic centres", such as shopping centres holding Boxing Day and New Year sales.
He went on to say: "On Christmas Day, let's be defiant in the face of terror and vigilant but also remember - we're talking about the birth of Jesus - we talking about peace - we must pray for peace and we must pray for our enemies."
Click here to listen to Anthony McRoy speaking with Premier's Alex Williams: