The December Christmas observance began as a pagan holiday that was “papered over” with Christian meaning when the Roman Empire officially converted to Christianity in the 4th century.
Early Christians refused to recognize it because:
1) they knew Jesus wasn’t born on December 25, and
2) nearly all the signs and symbols associated with Christmas had recognizable pagan origins.
It took about 200 years for it to gain acceptance and be viewed by Christians as a celebration of the Lord’s birth.
According to 1 Cor. 4:5 the Lord judges us by the motives of our heart, because that’s where sin begins (Matt. 15:19). He knows whether we’re focused on expressing our gratitude for the gift of His Son or indulging in pagan revelry. So as long as our hearts are right we’re not sinning by celebrating Christmas.
The idea that a Christmas tree is wrong comes from Jeremiah 10:1-5 where the Lord was poking fun at the pagans who worshiped evergreen trees that they cut down, brought into their houses, and decorated. He ended by saying that such things can neither do you harm nor good.