The relevant quotes in order:
The Illusions of German Ideology
The entire body of German philosophical criticism from Strauss to Stirner is confined to criticism of
religious conceptions. The critics started from real religion and actual theology.
The Young Hegelians are in agreement with the Old Hegelians in
their belief in the rule of religion, of concepts, of a universal principle in the existing world. [...]
First Premises of Materialist Method
The premises from which we [Marx and Engels] begin are not arbitrary ones, not dogmas, but real premises from which
abstraction can only be made in the imagination. They are the real individuals, their activity and the
material conditions under which they live, both those which they find already existing and those
produced by their activity.
In direct contrast to German philosophy which descends from heaven to earth, here we ascend from earth
to heaven. That is to say, we do not set out from what men say, imagine, conceive, nor from men as
narrated, thought of, imagined, conceived, in order to arrive at men in the flesh. We set out from real,
active men, and on the basis of their real life-process we demonstrate the development of the ideological
reflexes and echoes of this life-process.
So the text gives plenty of context and an explanation of the heaven and earth quote. Not sure if that is enough to answer the question.
Maybe to explain a bit more: Apparently there has been German writers in that time popular for coming up with new ideas (Strauss to Stirner, "young Hegelians"). And Marx and Engels try to explain that their own theories are also new, but do not follow in the footsteps of those other popular writers, but are entirely different. And the difference is supposed to be that Marx and Engels look at history, economics and politics as a result of material life, from which ideology was formed, as opposed to assuming that changes in ideology and religion over time shaped differences in politics, economics and history.