Precisely, what they are doing is as absurd as saying that our Lord wasn't crucified and that He didn't then rise from the dead and ascend into Heaven.
In light of this, as the letter writer conclude, they must leave Christianity and our God and create a new god that agrees with them.
Correct, although their current efforts seem to be along the lines of redefining marriage. They seem to think that the sin of homosexuality is nothing more than fornication outside of marriage, so they seek to redefine marriage to fit their agenda.
Nothing to disagree with there