Text 13 

jneyam yat tat pravaksyami
yaj jnatvamritam asnute
anadi mat-param brahma
na sat tan nasad ucyate

Translation

I shall now explain the knowable, knowing which you will taste the eternal. Brahman, the spirit, beginningless and subordinate to Me, lies beyond the cause and effect of this material world. 

Commentary by Srila Prabhupada 

The Lord has explained the field of activities and the knower of the field. He has also explained the process of knowing the knower of the field of activities. Now He begins to explain the knowable, first the soul and then the Supersoul. By knowledge of the knower, both the soul and the Supersoul, one can relish the nectar of life. As explained in the Second Chapter, the living entity is eternal. This is also confirmed here. There is no specific date at which the jiva was born. Nor can anyone trace out the history of the jivatma’s manifestation from the Supreme Lord. Therefore it is beginningless. The Vedic literature confirms this: na jayate mriyate va vipascit (Katha Upanishad 1.2.18). The knower of the body is never born and never dies, and he is full of knowledge. 

The Supreme Lord as the Supersoul is also stated in the Vedic literature (Shvetasvatara Upanishad 6.16) to be pradhana-kshetrajna-patir gunesah, the chief knower of the body and the master of the three modes of material nature. In the smriti it is said, dasa-bhuto harer eva nanyasvaiva kadacana. The living entities are eternally in the service of the Supreme Lord. This is also confirmed by Lord Caitanya in His teachings. Therefore the description of Brahman mentioned in this verse is in relation to the individual soul, and when the word Brahman is applied to the living entity, it is to be understood that he is vijnana-brahma as opposed to ananda-brahma. Ananda-brahma is the Supreme Brahman Personality of Godhead.

Commentary by Sri Visvanatha Cakravarti Thakur

Thus, by the methods mentioned above, one should know the jiva and paramatma. The paramatma however is present in all beings and is known as brahman. That brahman is worshiped by the devotees as personal, with qualities, and by the jnanis as impersonal, with no qualities. As the object of meditation with four hands situated within the body, brahman is known as paramatma. This verse speaks of that brahman first.

That form has no beginning (anadi). That means that since it is his svarupa, it is eternal. Mat param means "of which I alone am the supreme shelter." What is it? It is called brahman of which I alone am the supreme shelter, which is beyond cause and effect (na sad na asad). The Lord will later say brahmano hi pratistham: I am the basis of brahman.