It is the heart that God wants us to understand. By understanding the heart, we can understand why we suffer or have joy. As you continue to read the verses, think of how it is related to the heart, that God wants us to understand.
Interpretation:
God is preparing the early followers for the performance of the Joyous Service. They must understand the heart to purify their minds; and they must spread the truth to others before the Joyous Service becomes effective.
OFUDESAKI-GOD'S TEACHINGS 