I read that, according to a church tradition, the rich young ruler was Mark, the author of a gospel and disciple of Peter. I have not been able to find which church father(s) referenced that. Anybody know where or who that comes from?
You piqued my interest so I've spent a bit of time today looking into the question. I have not found a first hand source of this tradition but I suspect it to be Alexandrian, because the Coptic church places a special emphasis on the evangelistic work of St. Mark and is prone to read him into such passages. What I found interesting is that we have arrived at the title 'rich young ruler' by compiling bits of similar stories from all 3 Synoptic Gospels. But what if the same individual is not at the root of each story? Now the possibilities become very interesting. I've run across 3 main suggestions in the day's reading: Mark, Joseph of Arimathea, and Saul of Tarsus. But if we eliminate one or more of the criterion possibilities such as Barnabas and Nicodemos emerge. What you will find a lot of is milquetoast devotional sermons trying to wring some point about social justice or the evils of wealth out of the passage.
I too have heard that but there does not appear to be any substantial evidence to support it. Mark was more likely the son of the family where Jesus and the twelve had the last supper, but even this is reasonable conjecture rather than clear fact.