Santa claus can be seen, and children work out that it's their parents buying their gifts. God's an invisible spirit, and I guess because we create, and can bring about order, then believing a bigger force from which we came having a mind or consciousness, (a Creator) isn't to far a leap.

God is simply a more difficult concept to dismiss. * The concept of Santa Claus can be dismissed with increase in relevant knowledge and/or with observation. * A Creator God is much more difficult to dismiss. The possibility of an intelligent creator of "the Universe" or of "all nature" is not something we - as a species - have sufficient knowledge to dismiss as impossible or even implausible. Even philosophers debate the issue, without resolution.

Santa is tested every Christmas, he is firmly rooted in the temporal sphere. Children know their parents and home, they learn there is a bigger world out there and their parents tell them there a good things as well as dangers. As children learn more about the physical reality and the make-believe of magic tricks they realise Santa is in the make-believe but notice they still enjoy, participate and perpetuate the Santa myth for their own children. God on the other hand is not tested in the temporal sphere indeed he stands in as an explanation for the existence of the physical world. Notice all the explanations for the world require faith because we can't test the beginning of time a space. Many people don't like the idea of an all seeing God who might judge us for our bad behaviour. We all want to be free to do what we like in private. This is seen as a way to escape responsibility. Even in a perfect world we would get bored. The temptation to take what we want is a powerful urge even though we would be outraged if somebody took our stuff. God stands above that temptation. Things do get found out and we either choose order or give up any hope of climbing out of unregulated chaos. Believing is God is short hand for believing in order, the possibility of learning and improving life. For many people that's just Law and Justice but any lawyer will tell you the legal system is not about justice but due process. A competing struggle around what can be discovered, defined and proven to a specified level. God is not defined and yet we all have a sense of justice, we all hope for better and we all need forgiveness. We also need a great purpose for being alive than just getting older. God answers those questions for many people.