"It is very easy to inveigh against slavery and similar things in general terms, and to give vent to high moral indignation at such infamies. Unfortunately all that this conveys is only what everyone knows, namely, that these institutions of antiquity are no longer in accord with our present conditions and our sentiments, which these conditions determine. But it does not tell us one word as to how these institutions arose, why they existed, and what role they played in history."
I agree with Marv's point. Just to clarify context, Engels was dealing here with a professor who claimed to have revolutionized all sciences and developed a full-fledge philosophical and moral system, and whose ideas were gaining adepts, confusing workers and socialists.
Engels did not deny that morality played a role in the political struggle. Here, he is just making the point that moral denounciations can only express the conflict between the status quo and our human needs and expectations, without explaining such conflict. If we are going to successfully fight against the status quo, we need to go beyond that.