Diamond Sutra, Chapter 9
Buddha then asked, "Does someone who has started the path, a Stream-Enterer, say 'I have entered the stream'?" "No,", Subhuti replied. "A true disciple entering the stream would not think of themselves as an individual that could be entering anything. Only that disciple who can themselves from others and others in themselves can truly be called a Stream-Enterer." Buddha continued, "Does a disciple who is at the second stage, a once-returner, say, 'I am entitled to the rewards of a Once-Returner.'?" "No. 'Once-Returner' is only a name. There is no passing away, or coming into, existence. Only one who realizes this can really be called a Once-Returner." "Subhuti, does a venerable One who will never more be reborn, a Non-Returner say to himself, 'I am entitled to the rewards of a Non-returner.'?" "No. A 'Non-returner' is merely a name. There is actually no one returning and no one not-returning." "Does a Buddha say to himself, 'I have obtained Enlightenment.'?" "No. There is no such thing as Perfect Enlightenment to obtain. If a Perfectly Enlightened Buddha were to say to himself, 'I am enlightened' he would be admitting there is an individual person. Enlightenment consists fo transcending the Self and realizing that individuality is an illusion." Subhuti then said, "You have said that I, Subhuti, excel among your disciples in knowing the bliss of Enlightenment. But I do not say to myself that I am so, for if I ever thought of myself as such then it would not be true that I escaped ego delusion. I know that in truth there is no Subhuti and so Subhuti abides nowhere."
The Buddha is essentially saying that if we go around saying, “I am Enlightened,” it probably isn't true. Stream-enterer and the other are considered the levels of Enlightenment. In Ch'an Buddhism Enlightenment is divided into stages that the practitioner enters over time. These stages are described as: Stream Enterer, Once-returner, Non-returner, and Arhat.