Great masters go beyond their personality when they hit the highest point of self realisation and enlightenment even as a river loses its individuality and gains totality when it merges with the ocean. Krishna, representing the Truth, transcends the historical present and the Gita teaching owes its profound level of perfection and relevance for all times to the fact that it is spoken from the standpoint of the Impersonal Supreme Brahman, pointed out Srimati Sunanda in a discourse. The instruction is for every being to raise oneself towards a higher level of realisation of truth.
God, a guru, or the sastras can show the way and it is for every being to make the effort to tread the spiritual path. The Supreme Brahman pervades the entire created universe and also protects it by His Divine Sankalpa. Brahman remains as the final destination for all as well. So, Brahman is the essential principle enabling consolidated activities and experiences in life. The sun, the moon, the stars, etc, owe their radiance and power to illumine to Brahman. So too is the brilliance and heat in fire and the potency in water. He is the Omkara in the Vedas, sound in space and the atma in beings endowing all created objects and beings with specific functions and faculties. This truth can be grasped by observing and meditating on the external universe and also by looking inwards into oneself.
Unity and diversity are derived from the Supreme Brahman. The Oneness is the Truth while the diversity is the manifestation and cannot be called an illusion. A bhakta visiting temples to seek God who resides there in the form of the deity also sees the same God as the essence in all creation, including his self. Seeking the truth of the atma leads one to a state when he is not affected by any sorrow of any kind.