Here is a summary of the talk "Yoga of the Supreme Spirit" by Swami Nirmalananda Giri on the 15th chapter of the Bhagavad Gita, listing the main points:

1. The Eternal Ashwattha Tree: Krishna describes the cosmos as an eternal Ashwattha tree with roots above (in the divine) and branches below (various worlds and systems). The tree symbolizes creation, emanating from God, the root of all being.
2. The Vedic Hymns as Leaves: The leaves of the tree represent the Vedic hymns, vital to the life of the cosmos. These hymns, perceived by ancient Rishis (seers), connect to the subtle energy (Shakti) of existence and strengthen the cosmos when recited.
3. Deities as Principles: Vedic deities (e.g., Agni as fire) are not merely individual entities but represent deeper principles of existence. Reciting hymns invokes these powers, aligning and empowering both the cosmos and the individual.
4. Healing and Corrective Power of the Vedas: The Vedas are described as healing, empowering, and correcting imbalances in the cosmic vibratory pattern. Their neglect today is seen as a great loss to life and evolution.
5. Support for Vedic Practices: The speaker encourages supporting institutions and individuals who preserve Vedic traditions, such as reciting hymns and performing fire sacrifices (homam), as these acts benefit all beings and the cosmos.
6. The Vedas as Brahmic Sound: The Vedas are equated with the infinite sound of Brahman (God), a literal and profound truth, not just poetic praise. Associating with Vedic-centered groups ensures alignment with this divine "vibratory sound."
7. Cosmic Interrelation and Karma: The tree’s roots in Brahman connect to human action (karma) below. Karma yoga can lead to enlightenment because all levels of existence are interrelated, like pulling a root that moves the whole plant.
8. Non-Attachment as Liberation: The form of the cosmic tree is imperceptible in this world, with no beginning, end, or foundation visible. Liberation comes through non-attachment, cutting off ignorance at its root, not through mere aspiration.
9. Refuge in the Absolute: True refuge lies in the primeval Purusha (consciousness), not in philosophy or relative knowledge. This refuge frees one from pride, delusion, and attachment, ending the cycle of rebirth.
10. Constant Dwelling in the Self: Liberation requires constant dwelling in the Self through practices like japa (mantra repetition) and meditation, not sporadic efforts or self-delusion. Desires and opposites (pleasure and pain) must be dispelled.
11. Personal Anecdotes and Examples: The speaker uses stories—like a child’s death in Lahiri Mahasaya’s family and his own experiences in India—to illustrate detachment, duty (dharma), and the power of touching Brahman through practice.
12. The Eternal Goal: The talk concludes with the promise of reaching the eternal goal—home beyond the ocean of samsara (cycle of birth and death)—where one truly belongs, realized through Vedic wisdom and practice.

We recommend his book, The Bhagavad Gita for Awakening, for further exploration of these themes.