A quarter of British Christians do not believe in the resurection

[Angrboda]

You speak of "the definition" as if it exists independent of the people who claim themselves to be Christian.  It's not.  Christianity is what the self-professed Christian says it is.  If 1,000 years from now, no Christians believed in the resurrection, your opinion would simply be regarded as a relic.

It does exist independently! When God himself comes to earth and preaches a clear message for three years, how much more objective can you get? John 14:6 - Jesus answered, “I am the way and the truth and the life. No one comes to the Father except through Me." If 1000 years from now, no one believed in the resurrection, there would be no Christians.

Christians have been retconning scripture from day one. Your declaration means nothing in the face of the history of continuous reconstruction of your sources. But I see you yet again appealing to the No True Christians fallacy. They might not be Christians in your eyes, but that just underscores the subjectivity of your opinion. The 'truth' of such biblical passages is whatever self-proclaimed Christians say it is, and that is constantly changing.

“I have become convinced that we must put an end to atonement theology or there will be no future for the Christian faith. ”
― John Shelby Spong

"We cannot understand without wanting to understand, that is, without wanting to let something be said. It would be an inadmissible abstraction to contend that we must first have achieved a contemporaneousness with the author or the original reader by means of a reconstruction of his historical horizon before we could begin to grasp the meaning of what is said. A kind of anticipation of meaning guides the effort to understand from the very beginning."
― Hans-Georg Gadamer