In his introduction to the Penguin edition of the Communist Manifesto Gareth Stedman Jones opines Marxism is "a powerful and organized post-Christian religion that, in the name of science, addressed itself to the oppressed". How do you respond to this criticism?
The response is simply to show that Marxism is scientific. Did he offer any proof of how Marxist analysis is unscientific?
Why should a declaration of communism have placed such emphasis upon the world-transforming achievements of the 'bourgeoisie'?
The transformation of the mode of production by the bourgeoisie is emphasized because it is what brought about the existence of the modern proletariat. "What the bourgeoisie therefore produces, above all, are its own grave-diggers."
Why should it have been imagined that existing social and political systems were unreformable or that periodic economic crises were signs of the impending end of the property system as a whole?
It's not 'imagined', it's proved by a scientific analysis of human history and society.
Why should it have been assumed that there was a particular affinity bt the grievances of workers and the goals of communism?
Obviously because the goals of communism adress the grievances of workers- their exploitation and oppression.
Why should it have been believed that a historical process, governed not by ideals but by the clash of materially contending interests ('the class struggle'), would nevertheless deliver such a morally desirable result?
The inevitable result wasn't believed, it was discovered. It's up to you to decide if you think it's a morally desirable result- if you do, you'll become a Marxist and a communist.
"We stand with great emotion before the millions who gave their lives for the world communist movement, the invincible revolutionaries of the heroic proletarian history, before the uprisings of working men and women and poor farmers – the mass creators of history.
Their example vindicates human existence."
- from 'Statement of the Central Committee of the KKE (On the 90th anniversary of the Great October Socialist Revolution in Russia 1917)'