Kol Hamaarich B'Echad 5690 is a ma'amar by the Rebbe Rayatz that answers this question in detail.
Summary
"Unifying Hashem" means perceiving His Oneness. The specific point is that Hashem is both transcendant, but immanent in creation in the sense that His Essence is the vitaly of our very life, as well as the world at large.
This perception should be empowered by the innate knowledge of His Oneness in our soul, combined with the results of Torah study, but it is expressed intuitively, having integrated it into one's heart, perfected one's character thereby, and then actualized with the performance of mitsvot flowing from this total integration of mind, body and soul.
With this total integration, one is able to perceive His Oneness in everything, and at all times (especially when reciting Shema). The mitsvot one performs then reveal His transcendant Essence within the creation.
Basically, it is the intuitive perception that Hashem is One with His creation - even though the creation hides Him, it doesn't obscure Him in any way of consequence, similar to how the body that our soul enlivens isn't of consequence - the enlivening vitality that is our soul is who we are and of consequence.
Walk through of the ma'amar
- Hashem transcends nature, and that is why we call Him the Ineffable Name, because He is above time and space, and nature completely1.
- Hashem is the source of all creation, and the vitality of it2.
- When He actualizes creation, we call Him Elokim. The analogy is given of the core (essence) of the sun being shielded by the outer layers, so that it doesn't utterly scorch the earth, and we can benefit from its light; i.e. the actualization of creation is done in such a way that shields the direct, nullifying Light of Hashem, in Essence3.
- All the shield of Elokim does is enable the Light of Hashem to be perceived by the creation, but it doesn't change His Light in any other way, which is still present within creation as well, and this is what the phrase "Hashem is Elokim" means (as found in Shema, and Devarim 4:39 etc).
- Perceiving this deeply and intuitively is the mitsva of Unifying Hashem, and is done especially when we say Shema, where we declare "Hashem Elokienu, Hashem Echad - Hashem is our God, Hashem is One". I.e. Elokim, Who is actualizing creation, isn't separate from the vitality of the creation: Hashem. I.e. Hashem is One.
- This Oneness takes intellectual study and contemplation, many levels of it4, including studying how He is one with us, and how He is one with creation - and this study is part of the mitsva of Yediyat Hashem5.
- We are meant to study how our own soul is an analogy: paying attention to how our soul is the vitality of the body, and without it the body has an eye but can't see, has an ear but can't hear - the vitality of the body is our real life and what is of consequence about us, not the body itself, and use this as a springboard for understanding how Hashem is Elokim6.
- Another point of studying to perceive how He is unified with us is in order to understand that, just as we are a microcosm of creation, He is also unifed with creation as a whole7. We indeed grow to perceive Him in creation.
- As our soul has this knowledge, we can contemplate that which we've mastered with our intellect, with our heart, probing our instinctive sense of this Oneness in our Divine soul8.
- This contemplation of the heart is ultimately to transform our character traits to produce every fine quality in all our limbs, filtering down to and producing our good deeds.
- Through the full integration of heart and its corrected character traits, borne from this Torah study and the soul's knowledge of Hashem's Unity, one is able to in theory perceive Hashem's Unity with his entire being, at all times.
- Putting this theory into practice, into actuality through our heartful deeds, Hashem's Light is indeed revealed in creation, Elokim is revealed as Hashem, and He attains a dwelling, with Him being fully espoused and at home with the Jew performing the deeds. Through these perfected deeds, we truly sense that He is the vitality of our soul.
- This is the fulfilment of the Divine desire to have a dwelling place in the lowest world9, and this is the true meaning of Hashem is Elokim, i.e. Hashem is One. Performance of this mitsva achieves Hashem's very purpose of creation.
- As this is a life time's study and area of growth, every rung on the ladder of this knowledge and perception of Hashem's Oneness is a fulfilment of the mitsva of Unifying Hashem, which every Jew can do on their level.
- There are many other fine points in this maamar that warrant full study10
1. See Zohar III:257b
2. See Torah Or 55b
3. See Tanya Shaar Hayichud VeHaEmuna; Tehillim 84:12
4. Maamar Veyadata is recommended.
5. Derech Mitzvotecha Haamanut Elokut 1
6. Based on Iyov 19:26; Devarim 4:39
7. Avot D'Rebbi Natan 31
8. See Tanya Ch. 2, based on Iyov 11:7
9. Midrash Tanchuma, Nasso 16
10. Including how it explains why there is a discussion in the gemara of not extending the dalet, and insodoing not refrain from extending the chet, of the word Echad; why Elokim comes first, before Hashem Elokim in the Maaseh Bereshit, and how His justice preceded the fusion of justice and mercy; how Elokim represents evening, and Hashem's Ineffable Name represents the Light that comes in the morning; how one should contemplate Hashem's blessings and how He feeds the whole world and how this leads to one better understanding His Oneness; how one should realise that one is sent down to this world only to serve Him by fulfilling this desire for a dwelling down here, and one only has an alloted time to have the privilege of performing this service for Him, etc
