What is the Self, and is it God?

As pure water poured into pure water becomes the very same, so does the Self of the illumined man or woman verily become one with the Godhead.
– Katha Upanishad

For some reason, it is very difficult for us to accept our divine nature. This has always puzzled me. We pay money for books about how destructive we are. We stand in line to see movies that emphasize our capacity for making trouble. Then, when Jesus comes to tell us that the kingdom of heaven is within us, we say, “There must be some mistake.”

It is to convince us that our real Self is always pure and eternal that men and women of God keep arising among us. More than anything, we need to hear their good news that the source of all joy and security is right within. In the Hindu scriptures there is a precise term for our real nature: the Atman. All it means is “the Self” – not the little self, the changing personality with which most of us identify, but the higher Self, our real, changeless personality.

--Eknath Easwaran

Is "God" only within me, or is It just as much outside as inside me?

If by "me," I mean my organism or body-mind, then God is both inside and outside me. If by "me," I mean the fullest and most real me, then I, in the fullest and most real sense, am God. And so, I might add, are you. At least this is what I understand many of the great or reputedly great sages to be saying, whether or not any of them happen to be Christian.

Is there such a thing as a Christian sage? If Christianity says that God and I are not the same, but the truth is that God and I are the same and that a sage knows this fundamental truth, can any true Christian be a sage?

Yet, do I understand these sages correctly, or am I distorting their message? And, if do understand them correctly, are they correct, and, if so, how do I know that they are? By doing what they do until I experience what they experience? If I try this and succeed, will I know that "I and the Father are One"? If so, how will my life be different? How will I be different? How will the world be different because I am different?