tons of evidence Christianity existed before 325 AD, so... can't really swallow this.
fwiw, I think a lot of the reasons you have overlap in ancient religions is not just the trading of ideas but the generally shared experiences of early human cultures. things like flood myths make a lot more sense when you realize that most early civilizations were built on fuging flood plains.
Yes, Christianity was definitely around long before 325AD. Now there were a ton of different versions that varied rather dramatically but still they all believed in Jesus. Most didn't believe in the trinity.
Fun note: Some of the earliest church fathers Clement of Rome and Tertullian, some of the earliest sources for Christian authenticity of certain biblical cannons and beliefs believed the Phoenix really existed.
Yes the Pheonix
Let us consider that wonderful sign that takes place in eastern lands-that is, in Arabia and the countries around it. There is a certain bird that is called the phoenix. There is only one of its kind and it lives for 500 years. When the time of its dissolution draws near for it to die, it builds itself a nest of frankincence, myrrh, and other spices. When the time has come, it enters into the nest and dies. But as the flesh decays, a certain kind of worm is produced, which, being nourished by the essence of the dead bird, produces feathers" - Clement of Rome
"Take a most complete and unassailable symbol of our hope...I refer to the bird that is peculiar to the East. It is famous for its singularity, and is marvolous for its posthumous life. For it renews a life in a voluntary death. Its dying day is its birthday. For on that day, it departs and returns-once more a phoenix where just now there was none...Must men die once for all time while birds in Arabia are sure of a resurrection?" - Tertullian.