1) Christianity claims to be the 'Truth'. It claims that ONLY through the saviour can one find salvation. It states ABSOLUTELY that there is only one God and he IS the Truth.
That is Absolute Truth.
Immutability is a feature of absolutism.

2) "'Pure' Christianity" is a contradictio in adjecto - and a sign of Absolutist thinking. The Bible is riven with contradictions; the Old Testament is revoltingly violent and aggressive in places (many places - I shall be quoting them in new threads), while 'Jesus' is an obvious composite who preaches pacifism one moment, and the sword the next.

Atheism precedes Darwin (try reading de Sade if your Christian conscience will allow you!).

Paganism's history is one of assimilating other gods and traditions - that is true tolerance - unlike the 'one God only' bigotry of Judaism/Christianity/Islam.
Pagan polytheism, with its endless array of gods, GODDESSES, demigods and heroes, pays tribute to its openness.

It was Jehovah who said, "for I am a jealous God".