Kṛṣṇa Promises to Happen Again and Again
The next two verses are the essence of not only the whole Gītā but all the spiritual scriptures. Here He declares:
yadā yadā hi dharmasya glānir bhavati bhārata I abhyutthānam adharmasya tadātmānaṁ sṛjāmyaham II 4.7
paritrāṇāya sadhūnāṁ vināśāya ca duṣkṛtām I dharma saṁsthāpanārthāya sambhavāmi yuge yuge II 4.8
Whenever there is a decline in consciousness, whenever the collective unconsciousness increases, to establish the dharma, cosmic law or righteousness of the eternal consciousness, the science of eternal bliss, dharma saṁsthāpanārthāya, I Myself appear, age after age, sambhavāmi yuge yuge.
Kṛṣṇa declares, 'I come down again and again, to set right the wrong.' He keeps the generation of enlightened Masters open for the future, the possibility, the science of enlightenment open for the future generations on planet Earth. Only a courageous being, a complete Incarnation like Kṛṣṇa can declare this truth; sambhavāmi yuge yuge.
This verse is a promise; it is deliverance to all of humanity. This one single idea, when understood, can straightaway make you enlightened. There is a cycle of positivity and negativity, up and down, over time. Dharma, righteousness is restored eventually. This is Kṛṣṇa's promise.