Patanjali Yoga Sutra — sa eṣa pūrveṣāmapi guruḥ kālenānavacchedāt॥ ||1.26||

स एष पूर्वेषामपि गुरुः कालेनानवच्छेदात्||

īśvara is the ādyantarahita (Anādi — One who has no beginning). Hence, He is the Guru for even the ancient of all teachers.

Both yoga and sāṇkhya advocate Jīva, Jagat and īśvara. īśvara is the origin of the universe, but He has no origin. He is the cause of the universe, but He has no cause, say our scriptures.

Bhagavān reveals mysterious causes of His Divine Incarnation as under:

बहूनि मे व्यतीतानि जन्मानि तव चार्जुन।

तान्यहं वेद सर्वाणि न त्वं वेत्थ परन्तप॥गीता ४.५॥

bahūni me vyatītāni janmāni tava cārjuna|

tānyahaṁ veda sarvāṇi na tvaṁ vettha parantapa ॥Gītā 4.5॥

Arjuna, both of us have had innumerable births. I do remember all those incarnations but you do not.

यदा यदा हि धर्मस्य ग्लानिर्भवति भारत।

अभ्युत्थानमधर्मस्य तदात्मानं सृजाम्यहम् ॥गीता ४.७॥

yadā yadā hi dharmasya glānirbhavati bhārata|

abhyutthānamadharmasya tadātmānaṁ sr̥jāmyaham ॥Gītā 4.7॥

परित्राणाय साधूनां विनाशाय च दुष्कृताम्।

धर्मसंस्थापनार्थाय सम्भवामि युगे युगे॥गीता ४.८॥

paritrāṇāya sādhūnāṃ vināśāya ca duṣkṛtām|

dharmasaṃsthāpanārthāya sambhavāmi yuge yuge॥Gītā 4.8॥

Whenever there is deterioration of Dharma (virtuousness) and glorification of Adharma (viciousness), I take my incarnation and manifest in a visible form. I incarnate in different ages (Yugas) to defend and guard the virtuous people and to wipe out the wicked or malevolent. Thus, adharma is always under check by Bhagavān.

This is an inherent natural equilibrium or balancing principle brought out by Bhagavān, which we see in our ecosystem itself. In the wild, if herbivorous animals outnumber the carnivorous animals and threaten the very vegetation of the wild, the ecosystem quickly takes corrective action by increasing the number of carnivorous animals and vice versa. Thus balance is maintained (watch ‘Wolves change the river’ video on YouTube. Very interesting).

अजोऽपि सन्नव्ययात्मा भूतानामीश्वरोऽपि सन्।

प्रकृतिं स्वामधिष्ठाय सम्भवाम्यात्ममायया ॥गीता ४.६॥

ajo’pi sannavyayātmā bhūtānāmīśvaro’pi san|

prakr̥tiṁ svāmadhiṣṭhāya sambhavāmyātmamāyayā ॥Gītā 4.6॥

Despite being eternal, immortal or never-ending, and being the Supreme Lord of all creatures, I take different births owing to my innate mystifying, unfathomable, immeasurable power (Yogamāya).

न मे विदुः सुरगणाः प्रभवं न महर्षयः।

अहमादिर्हि देवानां महर्षीणां च सर्वशः॥१०.२॥

na me viduḥ suragaṇāḥ prabhavaṃ na maharṣayaḥ|

ahamādirhi devānāṃ maharṣīṇāṃ ca sarvaśaḥ॥Gītā 10.2॥

Both the myriad Gods and Maharṣis are ignorant of my antecedents. Because I am the unchanging cause (Timeless origin) of all of them.

इमं विवस्वते योगः प्रोक्तवानहमव्यम्।

विवस्वान्मनवे प्राह मनुरिक्ष्वाकवेऽब्रवीत् ॥गीता ४.१॥

imaṁ vivasvate yogaḥ proktavānahamavyam.

vivasvānmanave prāha manurikṣvākave’bravīt ॥Gītā 4.1॥

And thus, I taught this everlasting undisclosed Yoga to Sūrya, (in the beginning of Kalpa) and in turn, Sūrya imparted the secrets to his son Manu and from Manu to his son ikṣvāku Mahārāja.

यो मामजमनादिं च वेत्ति लोकमहेश्वरम्।

असम्मूढः स मर्त्येषु सर्वपापैः प्रमुच्यते॥गीता १०.३॥

yo māmajamanādiṃ ca vetti lokamaheśvaram|

asammūḍhaḥ sa martyeṣu sarvapāpaiḥ pramucyate॥Gītā 10.3॥

Mortals understanding Me as the ādyantarahita, beginningless, eternal, Timeless origin, Lord of the universe, would become omniscient (All-knowing) and be liberated from the clutches of all sins.