I have heard many Christian say that Santa Claus is part of their religion, and they must teach their children about Santa. This is not true.
Before we start, we must agree on some fundamental things. Firstly, we agree that Santa Claus is a lie. No reasonable adult will believe that there exists a fat man who speeds around the world in a sleigh powered by flying reindeers. Once we can agree on the obvious fact that Santa is a lie, then we can get to the biblical stuff.
Colossians 3:9-10 says, "Do not lie to each other, since you have taken off your old self with its practices and have put on the new self, which is being renewed in knowledge in the image of its Creator."
These verses alone state that Santa is sinful. Santa is a lie. Lies are sinful because they go against the Word of God, and hence Santa is sinful and un-Christian.
Some atheist parents agree but like to lie to their children about Santa. They believe then that Christian children should not be taught to give away this secret to other children and spoil Christmas for the atheist children. But the bible says otherwise.
Matthew 18:15 says, "If your brother sins against you, go and show him his fault, just between the two of you. If he listens to you, you have won your brother over."
James 5:19-20 says, "My brothers, if one of you should wander from the truth and someone should bring him back, remember this: Whoever turns a sinner from the error of his way will save him from death and cover over a multitude of sins."
Some Christians agree that lying is sin but they reason that we are all sinners and that we are not perfect. They also say that Jesus is forgiving and that Christians will be forgiven of our sins. This is not true. Consider Hebrews 10:26-27, which says, "If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, but only a fearful expectation of judgment and of raging fire that will consume the enemies of God."
Sure, sin is tempting, but we must try not to sin. Imagine if you're a woman and your husband saw a sexy girl and thought to himself, "I know I shouldn't sleep with her, but we all sin so never mind, I'll just sleep with her."
Some say that Christianity promotes love, faith, and generosity, which are values Santa teaches, so it's okay to preach about Santa. But Scientology teaches all those things, and I don't think your priest or pastor is going to recommend you tell your kids about that religion.