Shvetashvatara Upanishad
In the centre of the castle of the Infinite Spirit, which is Brahman, found in our own body, there is a small shrine shaped like a lotus flower in which there is a small space.

We need to find out who lives there and we should desire to know Him.

Why is it so important?

Because this little space within our hearts is as great as the whole vast world outside. Heaven, earth, fire, wind, the sun, moon and stars; whatever is and whatever is not, everything is there, for the whole universe is in Him and He dwells in our hearts.
The Spirit that lives in that space does not grow old and die. No one can ever kill the Spirit that is everlasting. The love of the universe dwells in the real castle of the Spirit, which we call Brahman.
The Self, the Atman, desires only what is real, and thinks nothing but what is true.

Just as the attendants of a king obey their master and are with him wherever he goes, so all love is truth and all thoughts of truth obey the Self.

If we depart this world not having found our Spirit and that love which is Truth, we do not realize liberation.

Chandogya Upanishad