"Belief in God is just wishful thinking"
Even if wishful thinking influences someone’s belief in God, it doesn't follow that Christianity is false. To falsify a claim, you have to examine the arguments and evidence in favor of it rather than discredit the motives of it’s adherents. For example, you can't disprove the existence of aliens simply by pointing out that many people believe in them out of wishful thinking. Aliens may or may not exist, but we can’t find out simply by examining the motives of people.
Furthermore, wishful thinking would give rise to a much different god than the one described in the Bible. People generally don't wish for strict moral laws, sexual restrictions, and the doctrine that many of their loved ones will suffer for eternity in Hell. Wishful thinking is more likely to result in atheism or new age gods that demand nothing from us. Unlike Christianity, these philosophies have the allure of living life any way you want, without eternal consequences. This type of freedom is much more appealing to the wishful thinker.