From what I've seen on the subject, the Romans didn't really have anything against the early Christians specifically... just anyone who was a nuisance to the state... so some Christians made the front page simply because they were a bit of a pain in their ass. Being tolerant of other belief systems was actually the Roman's more usual way of doing things to help ease subjugation of other peoples and nations. Sure, it was typically a case of, "Join us or die!" if they thought they could back up the threat, but those who joined weren't subsequently punished for their theology so long as they served the basic requirements of the empire such as paying taxes, supplying soldiers, obeying Roman rule, and so on. Even those who weren't treated very well weren't exploited based solely on religious reasons per se. It had more to do with age old reasons of politics, power, resources and poor (or occasionally bat-shit crazy) ruler-ship. Other reasons where religion may play a part in a Roman retaliation are that some cultures had religious leaders as their ruling class so it would make sense to remove them if they weren't playing ball. You might also punish a people by taking away something that they really want or like which could be religious based. Ultimately, there's quite some evidence that these stories of "Roman persecution of the Christians" were for the most part just propaganda perpetuated by the Christians themselves. After all, nothing makes a statement and gets people on your side like a good martyr or two. Anyway, it appears that Christianity grew organically (not really "underground" for very long) as it gradually replaced former Roman belief systems that failed to prevent the empire's decline to the point where Constantine was likely just leaning the way the wind blows to serve his needs. It's unknown if he was truly Christian in his heart (it's actually a pretty controversial topic) but it was definitely a smart and logical move to garner support from a popular movement and create a banner to rally around (along with a simple "us vs them" proposition based on religion) to defend/expand what remained of his empire.