Atman is not known by those who know it; it is known by those who do not know it. That is when one says he knows Ataman; he is saying it as a person.

Kenopanishad II – verses 1-3:-He who thinks that he knows really thereby proves himself ignorant. He who realizes that he cannot know Him has best understood Him. Those who seek to know Him, as they can grasp things of ordinary knowledge, can never achieve their object. Those who realize the limitation of the human mind in respect of the knowledge of the Supreme Spirit and, therefore, frankly confess ignorance, really approach a true understanding of it.

The limitation of human knowledge, when trying to comprehend the Supreme Being, is brought out in the above epigram.

Kenopanishad II – verses 4:-When it is known through every conscious state, it is rightly known and one attains eternal life thereby. Through his own self he gains strength and through his knowledge immortality. Not by reasoning but only by an awakening can we get a vision of the Supreme Spirit. Life, in relation to the Ultimate Reality, is like a state of sleep. Reason, in respect of Ultimate Reality, is like the impossible conception of a sleeping man trying to know what he is about, without waking up. As sleep is to waking, so is ordinary life to the state of realization.

If one thinks: "I know Atman well," then surely he knows but little of its form; he knows only its form as conditioned by man or by the concept of gods. Therefore Atman has to be grasped through inquiry, analysis and reasoning on the true base.

Atman(soul or consciousness) is not known by those who know it; it is known by those who do not know it.  That is when one says he knows Atman; he is saying it as a person. The Atman cannot be applied only to the individuality but to the whole experience (three states). But in reality the person and world created out of Atman, thus nothing exits other then Atman. Thus if one says he knows Atman as a person, he is making a wrong statement. One must know Atman as the formless clay of the three states (whole).

Atman is known when it is realized in every state of mind; for by such Knowledge one attains Immortality. By Atman one obtains strength; by Knowledge, Immortality

If a man knows Atman here, he then attains the non dual goal of his mission. If he does not know it here, he remains ignorance experiencing the cycle of birth, life and death as reality. Having realized the Self in every being, the wise relinquish the world and become immortal.