“One without a second” became the focus of the spiritual effort of Sannyasins and Yogis with the rise and development of the Vedantic tradition. The sense here was that the only thing important was to achieve oneness with the Absolute, and this meant abandoning the world, which, after all, was considered to be a distraction and an illusion, A one-pointed focus was needed so that the mind and senses were directed without any variance on the silent, unmoving, ineffable Brahman. Great Rishis taught the necessity and benefit of a complete renunciation of the world. Yet when one approaches the Kena Upanishad, there is no sign that this is the intention; on the contrary, it seems that “one without a second” needs to be understood in the light of the other dictum “all this is the Brahman.” The attainment of the spiritual experience of Oneness is intended to coincide with living a life in the world and interacting with the myriad individual forms and forces that bring about the creation.
Sri Aurobindo observes: “What then happens to the gods and the cosmos and all that the Lord develops in His being? Does it not all disappear? Is not the transfiguration of the gods even a mere secondary state through which we pass towards that culmination and which drops away from us as soon as we reach it? And with the disappearance of the gods and the cosmos does not the Lord too, the Master-Consciousness, disappear so that nothing is left but the one pure indeterminate Existence self-blissful in an eternal inaction and non-creation? Such was the conclusion of the later Vedanta in its extreme monistic form and such was the sense which it tried to read into all the Upanishads; but it must be recognised that in the language whether of the Isha or the Kena Upanishad there is absolutely nothing, not even a shade or a nuance pointing to it. If we want to find it there, we have to put it in by force; for the actual language used favours instead the conclusion of other Vedantic systems, which considered the goal to be the eternal joy of the soul in a Brahmaloka or world of the Brahman in which it is one with the infinite existence and yet in a sense still a soul able to enjoy differentiation in the oneness.”
Sri Aurobindo, The Upanishads, Kena Upanishad and analysis, pg. 107, 177-183