|Image above is from Answers in Genesis|
Aside from believing in the otherwise non-existent Santa Claus, there's also the problem of children that get excited for Christmas for wrong reasons. There are the parties and not to mention the toys where one might end up singing, "Joy to the world, the toys have come." instead of, "Joy to the world, the Lord is come." As a child who grew up in a non-Christian family, the idea of lying about Santa Claus was never considered a real lie and the idea of Christmas was all about toys never mind there are a lot of unfortunate people out there. When it's Christmas aside from the time I believed Santa Claus would give me toys if I were good all year (no wonder the average child believes in works salvation) but also, I was excited about going out to buy toys and receive toys.
There is nothing wrong about giving and receiving gifts on Christmas but the problem is when the people love the gift over the Giver. Ever since man fell down, the creation was worshiped instead of the Creator so it is normal for unsaved people to love the gift instead of the Giver. The attitude can also manifest when children can go sour on their parents because they didn't get the toys they wanted for Christmas. Then you can have children bragging about the toys they received during Christmas, get into some kind of unhealthy competition over who got better Christmas presents or who's been so good that "Santa Claus came" never mind the parents are tricking their children. Children getting deceived about Santa Claus being real when he is not is not cute at all.
Unfortunately this kind of attitude can persist even when children grow up to become adults. Forget about the whole idea that it's just a phase, they'll just outgrow it and let's just leave it alone. No, the Bible warns in Proverbs 22:6 says, "Train up a child the way he should go and when he is old, he shall not depart from it." Santa Claus has no place in that curriculum as much as there's no place for overindulgence and legalistic parenting. When children are just taught about Christmas is about toys and parties, they grow up to be materialistic. That kind of behavior persists because of a worldly view that wants Christmas but not Christ so they might as well just say "XMas" instead of "Christmas". XMas because X means an infinite variable where anything fits like Giftmas, Partymas, Clausmas (because children think of the non-existent Santa Claus instead of Jesus) or just anything because they don't want Christ. Sad to say but when the late heathen John Lennon said, "We're now more popular than Jesus Christ." he was right because Christianity is so unpopular to the world and they want anything but Jesus to be their Lord and Savior.
What people never think is that Christmas' real message has never about humanity's basic goodness but humanity's total depravity. Like it or not, people sin all they want because of the fact that they are not saved. The Lord Jesus Christ was born into the world as God becoming man as Matthew 1:21 declares that He will save His people from their sins. If humanity was not fallen into sin then there is no sin to save them from. The whole reason that Jesus is God becoming man was to live a perfect life pleasing to His Father and to save men from their sins.