That's not true. The majority of Christians voted for Biden – that's why he won by more than seven million votes. The majority or rightwing WHITE American evangelicals voted Republican.
Remember, that 70% of American Christians includes Black Americans, Indigenous Americans, Hispanic Americans, Asian Americans. etc. – most of whom ALSO profess to be Christians.
There is definitely something radically wrong in the cult of rightwing white American evangelicals. But, they are not the monolithic representation of Christianity in America of which you've convinced yourself. As you say, they supported the very worst of humanity – a man who is everything they ever claimed to abhor.
But, they are really only about a third of the country's population – and they vote every election, even if it's just for local dogcatcher. When you consider that most election, it' doing good to get HALF the electorate to cast ballots in the national elections, it's no wonder the cult has an outsized voice.
I admit, I too have been guilty of not voting in every election. I voted in presidential elections. But, before the Tea Party rose to power, I didn't vote in midterms. I vow, though, I will never miss an election again. Because, it was from those bottom up elections that the cult was able to rise to national power and so disrupt our nation's unity.