One of the biggest mistakes on the left is the confusion between solidarity and conformity. Solidarity does not mean a ticklist of prescribed beliefs.
The demand for conformity destroys solidarity.

Without critical thinking and new ideas, the left is dead. But some people march about with virtual clipboards, insisting that if you don’t tick every box, you don’t belong. Almost every day, I’m denounced for believing something I shouldn’t, or not believing something I should.

There are two kinds of disagreement: disagreement motivated by an open-minded search for where the truth might lie, and disagreement motivated by the shrill demand that you fall into line. The first kind is a pleasure, the second kind is horrible.

We need to be vibrant, heterodox, welcoming, broad and challenging. We pull together by embracing our differences, not purging them.