Today, there are many polytheists and neopagans who worship the ancient Roman gods. They are, however, a very new development, and not the descendants of an unbroken line stretching back to the Roman Republic. There was a long period (unless I'm wildly mistaken) when the gods of ancient Rome were fully dead.
As Christianity was rising, though, there was a significant amount of time, both before and after Christianity became the official religion on the Roman Empire, when Christians and pagans lived side-by-side. Some of this was peaceful, some of it was less so. But at a certain point, there were simply none of the latter group left, and Christianity had (aside from other religious minorities like the Jews) completed the conversion of the Empire, and of Europe.
When did that happen? What is the last reference we have to people worshiping Jupiter, Mars, Venus, etc? Preferably, what is the last reference that is generally accepted as valid, and not simply an accusation of scandalous pagan practices as a way to slander a Christian's name?