There is no difference between Brahman and Ātman; but this can be understood only through knowledge revealed by Upaniṣad-s. Brahma Sūtra (I.i.2) says, “janmādyasya yataḥ जन्माद्यस्य यतः”. This sūtra says that the cause of creation of this universe is Brahman, from whom, birth, sustenance and death originate. In other words, this universe and all its associated activities such as birth, sustenance and death originate from Brahman. Brahman is the cause and Brahman is also the effect and thus, in the whole of universe, there is nothing but Brahman. Taittirīya Upaniṣad (III.1) explains this aspect. It says, “yato vā imāni bhūtāni jāyante यतो वा इमानि भूतानि जायन्ते”. From Brahman these beings are born. Upaniṣad does not stop with this. It also says that these beings are sustained by Him and ultimately dissolves unto Him. The universe is not different from Brahman and due to illusion it appears as different from Brahman. The oft drawn example is a rope appearing as a snake. When it appears as a snake, there is fear and when the truth is known (it is only a rope), the fear goes away.
When Brahman is devoid of any form (nirguṇa Brahman – where nirguṇa means without attributes), we find that universe is full of different shapes and forms. That is why, Brahman is the cause and the manifested universe is the effect. Spiritual teachings alone do not explain Brahman; Brahman has to be experienced. How this experience is possible? Is there any way to expedite this experience? Yes, says Chāndogya Upaniṣad (VI.xiv.2). It says that one needs a person (Guru) to attain the right kind of knowledge. It also explains the quality of the teacher (Guru). “He must be a competent teacher. If he is unqualified and shows as if he is qualified, it leads to catastrophe for both the teacher and the student.” Upaniṣad specifically says that a blind man cannot lead another blind man. It is implied that only a Self-realized person can be a Guru to lead his disciples in the path of liberation. It is not the knowledge that alone is important, but it is the experience that counts while leading others. Therefore, Brahman can be realized only through proper teaching, explanations and examples by an experienced Guru. During the course of deliberating Brahman, everyone is bound to have too many doubts. It is natural in the process of learning. Guru should have knowledge as well as patience to clarify the doubts of his pupils.
Due to ignorance, one says that he will realize Brahman through sacrifices and rituals. Brahman is not bound, It cannot be sought in a sacrifice and It is not going to appear in a particular shape and form, as It already exists from time immemorial. It is omnipresent, omnipotent and omniscient. It is beyond sensory perceptions; It cannot be heard, It cannot be seen, etc. Therefore, by doing this or by doing that Brahman will appear, is a falsified notion. It is beyond the reach sensory perceptions. If It is beyond sensory perceptions, then what is the evidence, authority or proof for Its existence? This question can be answered by making counter questions to ourselves, such as how do we come into existence, how the planets function in a prescribed manner, why we are aging, etc. Going by the theory that for every effect, there has to be a cause, which is the cause for the material world? Is it Prakṛti or Nature? The answer again is no, as we do not know the source of Prakṛti, where all the actions take place. There has to be a cause for the existence of Prakṛti.
Taittirīya Upaniṣad (III.v) comes to our rescue by explaining a method to know Brahman. The Upaniṣad says, “tapasā brahma vijijñāsasva तपसा ब्रह्म विजिज्ञासस्व”. Tapasa means austerities, penance, etc; vijijñāsā means desire to know and willingness to inquire about; and brahma means Brahman. Upaniṣad emphasizes that one seeks Brahman, provided he has the desire and willingness to understand and realize It, by making self-sacrifices. Brahman is to be inquired within. It is to be searched and explored within and surely not outside the gross body. If It is not realized within, It cannot be realized outside the body. This Upaniṣad also says that knowledge required to realize It can be attained in stages. Śaṁkarācārya says, “Stage by stage, explore It within.” This goes to prove that one needs to train his mind and intellect (intellect is a superior form of mind), as Brahman can be realized only through these twin tools and not in any other way.
In knowing Brahman, we do not have a choice to make. If this object is better than the other one, we will go for the better object. We have the choice to decide as we could see and feel the objects. But, Brahman is not visible as He is subtler than the subtlest (Kaṭha Upaniṣad says that He is smaller than the smallest and bigger than the biggest). He cannot be seen or felt. He can only be realized through mind and not through senses.
What are the qualities of Self-realized persons? Śaṁkarācārya says that a realized person can be understood by his behaviour, expression of love for others and his compassionate attitude. The one who performs miracles is not a realized person, he is magician.
What is Self-realization? It is nothing but realizing our own consciousness. Brahman is explained as Pure Consciousness in Brahma Sūtra (III.ii.16). According to another school of thought (led by Kāśakṛtsna), Brahman Himself manifests individual souls. Thus there exists no difference between Brahman and the individual souls. Then, why we consider the Self and the self are different? There are two reasons. One is due to the spiritual ignorance known as māyā and another is due to upādhi (substitution – for example substituting the self for the Self). In fact, upādhi is also a product of māyā, which is the single important deterrent factor in Self-realization.