The heart is physically one of the most important organs of our body. If the heart is stopped the body will die. But what we read in today's proverb is not dealing with the physical heart. It speaks of the heart as the innermost region of our lives. The Hebrew mind looked at the heart as the central, spiritual, inward aspect of our souls. So when we look today at the issue of the heart and whether it is tranquil and strong - or whether it is tossed about and weak - means everything to the blessedness of the individual who struggles with issues of the heart.
The tranquil heart is the quiet one. What God is saying here is that this person's heart is strong and quiet. Their inner life is like the clear, mirror-like, state of the water in the early morning on a lake. This person's peace and calm is not disturbed by things that happen around it. The "soul-life" is strong and they can deal with problems and difficulties. There is an ultimate sense of peace here, because this one knows that God is sovereign over all things. They know they are in God's hand no matter what their circumstances look like. The Word of God is their comfort - and they will hold to it no matter what information in life seems to contradict it.
On the other hand there is a person who has "passion" in his life. The proverb tells us that this passion is like a rottenness in his bones. Things like jealousy, anger, over-zealousness, and envy run rampant in this one's heart. As a result there is no stability in him. These things run roughshod over his peace and contentment. There is a constant passion running wild within this man - almost pushing him from one extreme to another. And like a rottenness in his bones - he feels more and more like all stability and peace is gone from him.
One might wonder the source of such a peace. It is the knowledge of the gospel that brings us to peace with God. If there is peace between us and our God, then we are at peace with Him - and know peace in our souls. I've watched as strong believers have gone through the most trying of times, but they do so with tranquility. That is what the gospel of Jesus Christ looks like - and even more what it produces. If our sovereign God holds us and holds all the universe - we can trust Him.