Answer: One reason given was that the Bible tells us to celebrate the death, not the birth of Christ. Another reason mentioned was that Christmas is a pagan tradition. Another reason was Luke 16:15: “That which is highly esteemed among men is an abomination in the sight of God.”
But we should not “despise” nor “judge” those who don’t do as we do, realizing that “to his own Master [the Lord] he standeth or falleth….One man esteemeth one day above another: another esteemeth every day alike. Let every man be fully persuaded in his own mind. He that regardeth the day, regardeth it to the Lord, and he that regardeth not the day, to the Lord he doth not regard it” (Romans 14:3-6).
"Study to shew thyself approved unto God." 2 Timothy 2:15