How do we know the doctrine of the heart is true?

The truth of the doctrine concerning the heart is deeply rooted in scripture. Jesus teaches in Matthew 12:33-36 that a tree is known by its fruit, stating that what comes out of the mouth is indicative of the heart's condition. Proverbs 4:23 instructs us to guard our hearts because from it flow the issues of life. This underscores that our nature, whether good or evil, is reflected in our speech and actions. Thus, understanding the heart as the wellspring of morality and behavior aligns with both Old and New Testament teachings.