There's a lot of debate about whether Sol Invictus was a continuation of the cult of Elegabalus or if it was it's own cult that became popular after 274AD when Aurelian made it an official cult of Rome, so there's that. But what we do know is that while Elegabalus was killed rather savagely by his own people due to his belief, the Sol Invictus cult may have held a lot of influence on Rome. Over time after the promotion of the Solar cult, there's coins minted with inscriptions of reference to the Unconquored Sun, which shows a sign of social acceptance. There were several temples dedicated to the cult of Sol before and during Aurelian's reign. It's appearance and toleration in the Pantheon may have even influenced the coming overwhelming acceptance of Christianity in the next years which will see Constantine pass the Edict of Milan in 313AD. Constantine himself may have worshipped Sol Invictus himself before his total public conversion to Christianity. As far the Syrian reaction I cannot speak, but hopefully someone else can come along. This article online is pretty interesting on the subject as well, even though it's focus is on Christmas it provides valuable information. http://penelope.uchicago.edu/~grout/encyclopaedia_romana/calendar/invictus.html