Yes, we can still say it, Merry Christmas! Remember the last point of the Scout Law, being Reverent. Some synonyms for reverent are “adore, deify, glorify, revere, worship, venerate”.
A little background on the word Christmas is from the Middle English Christemasse, from Old English Cristes mæsse, literally, Christ’s mass. It’s first known use was before the 12th century.