(5) All living beings are pervaded by the same Buddha Nature

Infinite Life Sutra
V185. The ocean of the Tathagata’s wisdom is deep, expansive and immeasurable. Even the followers of the Hinayana cannot fathom it. Only Buddha’s can understand the wisdom of other Buddhas.

(Writers commentary) All such reality is contained within the Buddha, this Enlightenment(Buddha nature) is of one taste, this is to say that the omniscient all knowing Enlightened mind of all Buddha’s is one and the same, there being no difference between the Enlightenment of one Buddha from another, this one mind is our Buddha Nature(True Self).

Sokei-An writes:
“… you think that your soul is your own soul, different from my soul, and that God created each soul individually. We do not think so. We think all existence is soul, an ocean of soul, and we are the waves. This is the Buddhist’s basic concept of the universe.” (The Zen Eye, “The Universe is Another Name for Infinite Consciousness”, p. 31).

Nirvana Sutra
V165. The life of the Tathagata is the longest and most superior of longest lives. His eternal Dharma is the unsurpassed of all eternal things.” Bodhisattva Kasyapa said again to the Buddha: “O World-Honoured One! How did you, the Tathagata, gain eternal life?” The Buddha said to Bodhisattva Kasyapa: “O good man! There are eight great rivers, which are 1) Ganges, 2) Yamuna, 3) Sarabhu, 4) Ajitavati, 5) Mahi, 6) Indus, 7) Pasu, and 8) Sita. All these eight rivers and other small rivers drain into the great ocean. O Kasyapa! All the great rivers of life of all people, heaven, earth and sky drain into the Tathagata’s sea of life. Hence, the length of life of the Tathagata is incalculable. Also, next, O Kasyapa! As an illustration: it is like the case of Lake Anavatapta, which gives rise to four rivers. The same with the Tathagata. He gives rise to all long life-spans (emphasis added). O Kasyapa! As an example: of all eternal things, that of space is the foremost. The same is the case with the Tathagata. He is the foremost of all eternal things.

Nirvana Sutra:
V549. The Buddha said: “O good man! Litsten carefully, listen carefully! I shall now explain [it] to you. As an example: there is here a rich man. He has many cows, which are of various colours. He always has a man take care [of them] and bring them up [raise them]. This man, once on the occasion of a religious service, has all the cows milked and pours the milk into a container. The milk of all these cows is white. He sees this and is surprised. “The colour of these cows varies. How is it that the colour of the milk is white, all the same?” The man thinks and comes to realise that because of past karma and beings’ causal relations, the colour of the milk becomes one [and the same]. O good man! It is the same with all sravakas, pratyekabuddhas, and Bodhisattvas. That all of these possess the same Buddha-Nature is as with milk.
V550. Why so? The reason is that all “asravas” [defilements, taints, “influxes” of unwholesome thoughts and emotions] are done away with. And yet, there are differences between the Buddha, sravaka, pratyekabuddha, and Bodhisattva. Yet all sravakas and common mortals doubt and say: “How could it be that there can be no difference between the three vehicles?” All these beings, after a long time, come to understand that all three vehicles equally have the Buddha-Nature. The case is similar to that of the milk, which appears as it does because of past karma. Also, next, O good man! For example, when gold is treated [smelted] in a furnace, the dregs [scum] are discarded. Smelted again, gold appears and its price becomes inestimable. O good man! It is the same with sravakas, pratyekabuddhas and Bodhisattvas. All can arrive at the same Buddha-Nature. Why? Because the defilements have been done away with. It is like doing away with the scum from gold ingots. Hence, all beings have the same Buddha-Nature. And when one first hears of the hidden store of the Tathagata and when one later attains Buddhahood, one, in the course of time, comes to know this fact. This is as in the case of the milk of the rich man, in which the oneness of the quality of the milk becomes apparent. Why [this dawning of awareness]? Because innumerable billions of defilements have been done away with.”
“Again, Mañjuśrī, any of these performers who desire to be united with their own dhātu will secure for themselves a small degree of happiness, for they will become as they say. If they are born as a hermaphrodite, a heap of suffering will arise with that happiness. The tathāgata-garbha is present in women and likwise also in men. How could they secure that and become as they say by themselves ? Therefore, the dhātu of all beings is a single dhātu. [The Tathāgata] applied himself to the holy life all the while that he had not established himself therein and [eventually] arrived at tathāgatahood.”

V426. “Also, O good man! As an example, there is a medicine in the Himalayas called “pleasing taste”. It tastes very sweet. It grows hidden under a deep growth of plants, and we cannot easily see it. But from its scent, one can come to know the whereabouts of this medicine. In days gone by, there was a chakravartin who, placing wooden tubes here and there in the Himalayas, collected this medicine. When it had ripened, it flowed out and entered the tubes. It tasted truly right. When the king died, this medicine became sour, salty, sweet, bitter, or hot, or light. Thus, what is one, tastes differently according to the different places. The true taste of the medicine remains in the mountains; it is like the full moon. Any common mortal, sterile in virtue, may work hard, dig, and try, but cannot get it. Only a chakravartin, high in virtue, appearing in the world can arrive at the true value of this medicine because of happy circumstantial concatenations. The same is the case [here]. O good man! The taste of the hidden store of the Tathagata is also like this. Overspread by all the growths of defilement, the beings clad in ignorance cannot hope to see it. We speak of the “one taste”. This applies, for instance, to the Buddha-Nature. On account of the presence of defilement, several tastes appear, such as the realms of hell, animals, hungry pretas, devas, human beings, men, women, non-men, non-women, Kshatriya, Brahmin, Vaishya and Sudra.

 

(Commentary of this passage)
For Example say there is a man walking in the desert and in the distance he see’s a house so he decides to go the house he sees, upon arriving near the house he realizes that the house was actually a mirage and what he was really seeing was a Tree. In this analogy the Tree is the All pervading, unchanging absolute which is pure from the beginning and the house is Samsaric Adventious defilements/ignorance .
In truth the mirage of the house(samsara) and the Tree(The Source/Buddha Nature) are actually the same entity, yet the house(samsara) was nothing more than a mirage/illusion that was superimposed upon the Tree(Actual Source). Without the Source of the Tree the Mirage would have no basis or foundation to appear from. Thus the Mirage(Samsara) is by its own nature Non-existent, But due to it being seen(misapprehended) the Mirage is mistaken to be real and through this Mirage/illusion/Ignorance you get the source for the 12 links of dependent origination and the arising of Samsara and the Whole mass of suffering.

 

Awakening of Faith in Mahayana

Question: How is this to be corrected? Answer: In order to correct this error it should be understood that the Tathagata-garbha, from the beginning, contains only pure excellent qualities which, outnumbering the sands of the Ganges, are not independent of, severed from, or different from Suchness; that the soiled states of defilement which, outnumbering the sands of the Ganges, are not independent of, severed from, or different from Suchness; that the soiled states of defilement which, outnumbering the sands of the Ganges, merely exist in illusion; are, from the beginning, nonexistent; and from the beginningless beginning have never been united with the Tathagata-garbha.

Further explanation given in the Awakening of Faith in Mahayana text

 

Awakening of Faith in Mahayana

c. The Relationships between Enlightenment and Nonenlightenment Two relationships exist between the enlightened and nonenlightened states. They are “identity” and “nonidentity”.

(1) Identity Just as pieces of various kinds of pottery are of the same nature in that they are made of clay, so the various magic-like manifestations (maya) of both enlightenment (anasrava: nondefilement) and nonenlightenment (avidya: ignorance) are aspects of the same essence, Suchness. For this reason, it is said in a sutra that “all sentient beings intrinsically abide in eternity and are entered into nirvana. The state of enlightenment is not something that is to be acquired by practice or to be created. In the end, it is unobtainable [for it is given from the beginning].” Also it has no corporeal aspect that can be perceived as such. Any corporeal aspects [such as the marks of the Buddha] that are visible are magic-like products of Suchness manifested in accordance with the mentality of men in defilement. It is not, however, that these corporeal aspects which result from the suprarational functions of wisdom are of the nature of nonemptiness [i.e., substantial]; for wisdom has no aspects that can be perceived.

(2) Nonidentity Just as various pieces of pottery differ from each other, so differences exist between the state of enlightenment and that of nonenlightenment, and between the magic-like manifestations of Suchness manifested in accordance with the mentality of men in defilement, and those of men of ignorance who are defiled [i.e., blinded] as to the essential nature of Suchness.