The men were asked to leave by a bartender at the City Arms and after originally thinking it was a joke, the priests attempted to prove their profession before making for the exit.
As they were leaving they were approached by a member of staff who realised they were telling the truth. His apology was greeted by a round of applause from locals.
Speaking to WalesOnline, Matt Morgan, assistant manager at The City Arms, said: "When we realised our mistake we quickly apologised to the priests and thankfully they were all great sports and saw the funny side of the situation.
"We offered them a free round of drinks and they were happy to stay and continue their evening in the pub.
"Our Rev James ale is one of our most popular beers so it was great to have a real-life Reverend James and his fellow priests enjoying a pint or two!
In a statement, the Archdiocese of Cardiff said: "We'd like to thank the City Arms for being good sports through all of this and their kind gesture to our seminarians - and please note a number of our clergy, including the Archbishop of Cardiff, frequent your bar so don't turf any more out please!"