I taught my son that Santa was make believe from the start (since my parents lying to me about Santa was too traumatic in my situation) and my husband wasn't brought up believing in Santa. At four years old he didn't believe me because everyone else his age believed in Santa. I was OK with it because I knew that I had told him the truth and in the long run I am hoping he can know I am someone who is not going to lie to him.
At 19/20 years old I learned that Jesus' birthday was not actually December 25th, and that it was originally a Roman holiday celebrated by pagans. Again, I was upset that all those years even the church had lied to me. I was taught - growing up - by teachers, parents, Sunday School and so on that it is really important not to lie. So the whole situation was extremely upsetting.
People wanting to celebrate Jesus' birth on December 25th is a tradition, yet I don't believe it is right for the Church to tell people He was born then — or to withhold the truth. I personally have known churches/people who teach it is a sin to celebrate Christmas and I know people who teach it is a sin not to celebrate Christmas. I got tired of being pulled around by "every wind of doctrine" and for years now have held fast to Romans 14:5-6. I do know Christians who make the most of the Christmas Season to share the gospel with loved ones who would otherwise not be willing to listen. I also know people who do not celebrate Christmas and make the most of the holiday season taking opportunity to share their faith in Jesus.
Romans 14:5-6 King James Version (KJV)
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 unto the Lord; and he that regardeth not the day, to the Lord he doth not regard it. He that eateth, eateth to the Lord, for he giveth God thanks; and he that eateth not, to the Lord he eateth not, and giveth God thanks.
Mark 7:7-9 King James Version (KJV)
Howbeit in vain do they worship me, teaching for doctrines the commandments of men. For laying aside the commandment of God, ye hold the tradition of men, as the washing of pots and cups: and many other such like things ye do. And he said unto them, Full well ye reject the commandment of God, that ye may keep your own tradition.
Philippians 1:18 King James Version (KJV)
What then? notwithstanding, every way, whether in pretence, or in truth, Christ is preached; and I therein do rejoice, yea, and will rejoice.