usually it's best to ask someone what their beliefs are. while a lot of people dont get offended, some do. in situations where you're in public being nice to someone, and you say merry Christmas, theres usually no time to start a conversation and ask what their beliefs are but in situations where you know the person, or are trying to get to know them, or even having a conversation with them, its best to ask before you say the phrase to someone that observes another holiday. a lot of people try to push their beliefs on others and others are very passionate about what they believe and/or celebrate so its easy to see "merry christmas" the wrong way when you don't celebrate it.
