Perhaps the best way to expand on this most fundamental of all mystical modes of thought, driven by a conscious awareness of timelessness, is to turn to a quote by a well respected and very knowledgeable expert on the subject.
Ancient nondualism at its best, as we find it in the teachings of Buddha and Sankara, represents mysticism at its loftiest. Theistic, pantheistic, and other forms of mysticism are pale reflections thereof. The core of nondualistic mysticism lies in the conviction that the supreme truth is the timeless and formless Being or Nonbeing. Reality is essentially indefinable and nonverbal. It is beyond all opposites -- nondual. Beyond all limitations of cosmic expression, it is still the source and support of the cosmic manifold. ...
A Hindu theistic mystic may say, "I am at one with Krishna." Insofar as he is a theist, and not a nondualist in the strict sense of the word, he finds it difficult to rise above the Krishna-form. Indeterminable Being is for him only an abstraction. At best it is the limitless power and glory of the determinate Krishna-form. Similarly, a Christian mystic, who is usually a theist, says, "I am at one with Christ, or with God the Father, who is inseparable from Christ." He finds it extremely difficult to rise above the Christ-form and comprehend Being in its transcendental universal essence. ...
Formless Being is the common ground of all such specific symbols of the Divine, as Krishna, Buddha, Christ, Mohammed, Moses, etc., with all their historical particularities. ... The theist equates the historical form with the transhistorical. Consequently, the transhistorical reality appears to him as a person. Failure to rise above personalities and to comprehend the superpersonal divine ground introduces an element of parochialism into his religious outlook. He fails to appreciate the common universal essence of all religions.
Being, Evolution, and Immortality - An Outline of Integral Philosophy
1974, The Theosophical Publishing House