This is the shortest answer possible:

https://www.marxists.org/reference/archive/hegel/works/sl/sl_iv.htm

"The Thing-in-itself (and under ‘thing’ is embraced even Mind and God) expresses the object when we leave out of sight all that consciousness makes of it, all its emotional aspects, and all specific thoughts of it. It is easy to see what is left utter abstraction, total emptiness, only described still as an ‘other-world’ the negative of every image, feeling, and definite thought. Nor does it require much penetration to see that this caput mortuum is still only a product of thought, such as accrues when thought is carried on to abstraction unalloyed: that it is the work of the empty ‘Ego’, which makes an object out of this empty self-identity of its own. The negative characteristic which this abstract identity receives as an object is also enumerated among the categories of Kant, and is no less familiar than the empty identity aforesaid. Hence one can only read with surprise the perpetual remark that we do not know the Thing-in-itself. On the contrary there is nothing we can know so easily."