In plain words
Only when we experience this truth in actual experience do we become fit to recognise the divinity that is oneness.
For this we must first of all enquire into the principle called the three Advaitas. The three Advaitas are: Bhāva Advaita, Kriyā Advaita and Padārtha Advaita. When we enquire well into the principle of these three, we understand the feeling that everything is one.
There is the analogy of the cloth and the thread. That is, we must enquire into the analogy of the cloth and the thread. Now what is the substance in these two? It is the principle of one word, one thing, present in cause and effect. That is Advaita. The substance in this cloth and in the thread is cotton, one and the same. That is, recognising the oneness in the many: this is Bhāva Advaita. That is, only when we recognise what the principle in all human beings is will we understand this Bhāva Advaita. In cause and effect it must be the one thing. As I say many times,
The cattle are many, the milk is one; the living creatures are many, the jīva is one. The kinds of cloth are many, the stuff is one.
This itself is Bhāva Advaita.
Kriyā Advaita means this: in all the actions we do with purity of thought, word and deed, to set the fruit apart and experience it for the pleasure of the Paramātma; this is Kriyā Advaita. That is, to do our actions in the spirit of offering them to God: that is Kriyā Advaita.
Next, Vastu Advaita. That is, Padārtha Advaita. In the world we are able to see many forms and names. We are able to hear them. Though forms and names differ, in this all human beings and all living beings are one. The substance in all is the five prāṇas. The elements beginning with space. Whatever body we examine, we see no other elements in it than these five. Whatever earth is in this body, that same earth is in all these bodies. Whatever water is in this body, that same water is in all living beings. Whatever fire is in this body, that same fire is in all bodies. Whatever air is in this body, that same air is in all beings. Whatever sound we experience in this body, that same sound is in all living beings. So because the substances called the five elements are in all living beings, we call this simply Vastu Advaita.
So when we recognise Bhāva Advaita, Kriyā Advaita and Padārtha Advaita well, we understand clearly that everything is one. It is only on account of the differences of the vehicles that we look on things as separate.