It’s decades too late at this point, but not accepting the far-right fundamentalists who were spewing clearly non-Christ-like rhetoric as fellow Christians would have been a good start. This applies more to church leadership than lay people, although a general sense of not accepting them from everyone would have still helped. They were happy to consider them when counting how many people in the US were Christian. Unlike non-theists and theist non-Christians alike, all Christians gain a lot by being the de facto state religion despite it supposedly being illegal.