First post hashkafa post. Second a practical answer :
Hashkafa:
The concept of achdus Hashem as elucidated by the Alter Rebbe in shaar hayichud vhoemuna goes as follows:
The cannot be any separate existence other than Hashem (ain od milvado). So how do we experience a world where it does look that way?
Because Hashem hides this reality from us but by Him He was never separate from anything and is still one with it. So creation added nothing to His Oneness.
(A modern day example would be seeing the website, and not seeing that is really all letters, the exciting visuals area a facade)
So really everything is one with Hashem. Our job is just to reveal it. We reveal it by 1) doing mitzvos asei 2) using divrei rishus for Hashem.
The ultimate of all our work is when moshiach comes, and the facade that allowed us to have bechira chofshis
will be lifted, and all the G-dly energy we drew down through our mitzvos will be revealed in the world we refined.
Kedusha is anything batul to Hashem. Klipa / avoda zara is something that acts like it’s own power or entity – elokim acherim.
We are a microcosm of the world – and the same goals are expected of each of us –
To work on getting rid of lessening our own ego and making what Hashem wants we want. The more we do that, the more Hashem can dwell on us as it says about a Baal guava ain ani vhu yocholin ladur.
A person who completely conquers his yetzer hara and had no more ulterior motives / selfish desires, and only wants to fulfill Ratzon Hashem, is a tzadik and Hashem dwells on him to various degrees depending on how much of a tzadik he is. Some have ruach hakodesh, they aren’t bound by nature and can accomplish the impossible etc. because the more kadosh a person is, the more they can access these levels of kedusha which out them above the world – like tzadik gozer vHKBH mekayem.
A tzadik today is like the beis Hamikdash – Hashem dwells within and by visiting and being mekushar to a tzadik, it helps one grow in their Avodas Hashem.
So basically the root of avoda zara is arrogance, and anything kadosh is batul to Hashem. A tzadik can’t be avoda zara just like the beis Hamikdash isn’t avoda zara because his whole being is batul.
Ok lots here – each line can reference a whole
perek or more which is why I’m being scarce and not explanatory as I can’t write out whole prakim of Tanya. Abs maamarim etc. But you can ask for more if you want.