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, ponder how it is related to the heart.
Interpretation:
God is trying to help people understand the heart, so they can purify their minds by themselves. God is placing illness as guidance to do this.Because we are all Oyasama’s and God’s children, God loves everyone; and wants to save everyone from suffering.
Heaven's Truths, In Becoming Spirited 