I have been trying to do some research on some of the ancient religions that either share a similar era to early Christianity (like gnosticism and Mithras Mystery Cult) or others that people have caused to be linked to early Christianity (like Krishna).
I have found scholarly work incredibly hard to come by… and original documents nonexistent. Every person who writes on this (and there are thousands online) seem to quite literally copy and paste one another without any citation at all. As a researcher, I am dubious of this, though I admit there are probably times I make that same mistake.
I am still working on the Krishna page, but I think I have found a serious, factual and incredibly scholarly (if that is an appropriate adjective for scholarly) article about Mithras. Anyone interested in the Roman Mithras Mystery Cult should go and check out this link: http://www.tertullian.org/rpearse/mithras/display.php?page=main
The author also has a page with comments on the connections of http://www.tertullian.org/rpearse/mithras/display.php?page=Mithras_and_Jesus if that is of particular interest to anyone.