Because my people hath forgotten me; rather, Surely, etc.; or better still, Yet surely. It is not uncommon for a particle of asseveration to acquire a contrasting force from the context; see e.g. Jeremiah 3:20; Isaiah 53:4; and, still more completely parallel, Isaiah 2:6; Jeremiah 9:1, where Authorized Version, with substantial correctness, has "nevertheless."
Israel "forgot" Jehovah (as Jeremiah 2:32); no doubt he was responsible for so doing, but still it was not "of malice preponse." To vanity; i.e. to the unreal idol-gods. And they have caused them to stumble; viz.
the idol-gods; these are responsible (.for they have a real existence in the consciousness of their worshippers) for this interruption of Israel's spiritual progress. In their ways from the ancient paths.
"From," however, is interpolated by the Authorized Version; the Hebrew places "the ancient paths" in apposition to "their ways," "Stand ye in the ways," Jeremiah cried at an earlier period, "and see, and ask for the old paths, which is the good way" (Jeremiah 6:16).
These "old" or "ancient" paths were ideally "their ways," the ways appointed for the Jews to walk in. To walk in paths; rather, in tracks, footpaths leading up and down and often ending in nothing; or, in other terms, in a way not cast up (Isaiah 40:3, Isaiah 40:4, gives a graphic picture of the operation of "casting up a way").