Good point Alison! But atheism and being left wing are not synonymous. I'm a old fashioned lefty (by the way the Labour party isn't left wing - that may or may not be the reason why it isn't in power) but I am a Christian. We come in all shapes and sizes and, as I'm sure you know, all shades of political and theological opinion.
But I do agree, and I'm not sure who I'm agreeing with here, that robust can sound aggressive and is often counter productive, because if people feel bludgeoned by the tone of the debate / discussion, they will often withdraw from that debate. Calm reason, courteous listening and responding to all the points made, rather than ignoring very valid comments, by a person who is of a different opinion, is far more more likely to achieve mutual respect, mutual listening and mutual understanding, if not agreement.