A religion is only a form the divine spirit takes to manifest itself.
But no form can remain unchanged.
Christianity, which was born in the Middle East, took on certain elements in the beginning from Greek and Roman cultures. These elements were added to those inherited from the Jewish religion, which had itself been influenced by the religions of neighbouring countries: Egypt, Mesopotamia and so on.
A religion is never born from nothing; it receives certain elements from previous religions and itself transforms as it spreads further from its place of origin.
Although people always preserve and study the same sacred texts, the way they understand and experience these texts becomes more and more remote from what they are reading. Evolution is the law of life, which is why it is not reasonable to desperately try and perpetuate the form of any religion; we should only make sure that it is always alive with the spirit.