Bhagavad-gita is the essence of Vedic Knowledge.
The spirit of Bhagavad-gītā is mentioned in Bhagavad-gītā itself. It is just like this: if we want to take a particular medicine, then we have to follow the directions written on the label. We cannot take the medicine according to our own whim or the direction of a friend. It must be taken according to the directions on the label or the directions given by a physician. Similarly, Bhagavad-gītā should be taken or accepted as it is directed by the speaker himself. The speaker of Bhagavad-gītā is Lord Śrī Kṛṣṇa. He is mentioned on every page of Bhagavad-gītā as the Supreme Personality of Godhead, Bhagavān. Of course the word “bhagavān” sometimes refers to any powerful person or any powerful demigod, and certainly here Bhagavān designates Lord Śrī Kṛṣṇa as a great personality, but at the same time we should know that Lord Śrī Kṛṣṇa is the Supreme Personality of Godhead, as is confirmed by all great ācāryas (spiritual masters) like Śaṅkarācārya, Rāmānujācārya, Madhvācārya, Nimbārka Svāmī, Śrī Caitanya Mahāprabhu and many other authorities of Vedic knowledge in India. The Lord Himself also establishes Himself as the Supreme Personality of Godhead in the Bhagavad-gītā, and He is accepted as such in the Brahma-saṁhitā and all the Purāṇas, especially the Śrīmad-Bhāgavatam, known as the Bhāgavata Purāṇa (Kṛṣṇas tu bhagavān svayam). Therefore we should take Bhagavad-gītā as it is directed by the Personality of Godhead Himself.
(From Introduction to Bhagavad-gita)