It wasn’t until Constantine that Christians could practice more openly. Also, after Constantine is the first recording of Christmas being celebrated, it doesn’t mean that it wasn’t celebrated beforehand. Interestingly enough, our source for that - the Roman almanac in 336 - is older than the earliest source for sol invictus being on the 25th of December in the Chronography of 354. If anything, it could have very well been the pagans changing their dates to compete with Christians, which is what happened in Scandinavia when Yule was moved to the 25th of December to coincide with Christmas.