The Olympian pantheon was certainly in decline by the late Roman empire. There were many cults and new religions popping up, and one of the most prominent one was the cult worshiping the Sun - Sol Invictus. Every emperor from Aurelian up to Constantine the Great had Sol Invictus featured on their coins.

If you're talking about paganism in general then obviously not, almost all of the Germans (Germanic tribes, so Franks, Cimbrii, Goths, Saxons, Jutes etc etc) were still pagan, in fact following the WRE's fall the Saxons in England successfully reverted the country (or multiple countries that made up modern day England at the time) back to paganism, it remains one of the few places that this happened in. What Constantine saw, and as a cynic I think that he was just a shrewd politician, was that Christianity was not going away. With how intergrationist the Roman religion was, it was quite easy to take this new, upstart religion and incorporate it into the Roman way of life. Paganism was not necessarily in decline, but Christianity was certainly on the rise, and Constantine saw that and acted on it. Now, there's some thought that certain parts of Greece remained vaguely pagan even into the 700s, yet one shouldn't worry too much about that unless you're particularly interested.