Is believing in Jesus the same thing as believing in Santa Claus? It depends on what you mean by that. Jesus and Santa Claus (St Nicholas) both existed. So there is some similarity there.
But people usually mean something different. They mean these individuals in their supernatural roles. Is believing in a Jesus that performed miracles and rose from the dead the same as believing that jolly ol’ St Nick delivers toys to millions of children each year?
Let’s look at St Nicholas. He was a Christian bishop of Myra (modern Turkey) in the 4th century. None of the modern attributes of Santa Claus go back to his origins. Yes Nicholas gave presents, but it was providing money to prevent girls from being sold into prostitution.
What we know of as Santa Claus did not develop until much later. We get a bit closer with the creation of Father Christmas in the 16th century. Note that is 12 centuries later! But what we know as Santa really begins to appear in 1821 with the poem, Old Santeclaus with Much Delight, and in 1823 with A Visit From St Nicholas (The Night Before Christmas).
How does that compare with Jesus?
Realize that even much of what we know about Nicholas of Myra comes from later traditions and not near contemporary sources. For Jesus, we have biographies (note the multiple!) that are thirty to fifty years after the events, if not earlier. In those biographies we have all of the elements that we associate with Jesus. We don’t have to wait many centuries before Jesus develops into who we know today.
When we look at 1 Corinthians (written 20 years after the events), we get clear evidence of the crucifixion and resurrection of Jesus. Paul’s letters also include teachings about Jesus being divine. There is no evidence that there was a Jesus story without these elements or that they were added later.
So is believing in Jesus the same as believing in Santa Claus? Yes, if you only mean their existence. If you mean their identities as we understand them today, not at all. It took a long time for Nicholas of Myra to become Santa Claus. Jesus was understood as the divine Son of God, crucified for our sins and raised from the dead right from the beginning.