It is important to note that Engels had a different background from most of his associates, including Marx. First, he was not a Jew. Second, his social class could probably be described as “ultrabourgeois”. He loved dressing well, hunting, and had elegant manners. For the Jewish clique which was conspiring to create the virulent and antilogical ideas which would later become known as Marxism, a sophisticated non-Jew like Engels was a prize catch.
For his part, Engels was given to occasional anti-Semitic outbursts. Consider: “I begin to understand French anti-Semites when I see how the Polish Jews with German names everywhere worm themselves in, take liberties and everywhere push forward until they dominate public opinion in [Paris].”
Also, in an attitude typical of pan-Germanism, Engels was dismissive of the national aspirations of Slavic peoples. He considered the Slavs as a “people without a history” and to be incapable of self-government. “They are peoples who were either already under foreign rule when they entered into the first primitive phase of civilisation or who were actually forced into earliest phase of civilisation by their foreign masters.”
When it came to the active promotion of communism, it was Engels who led the way. Marx was often sick and at home whereas the political battles were fought by Engels.
If Engels had not been so self-effacing, it would most likely have been him and not Marx by whose name the new philosophy would be called.