"Deep calleth unto deep at the [noise] of thy waterfalls" (Psalm 42:7a). What can be deeper than the heart? Solomon, the wisest of men, advised us, saying: "Keep thy heart with all diligence; for out of it are the issues of life" (Proverbs 4:23). When a heart has heard the voice of God, it calls other hearts to hear and to respond.
This is exactly what brother Watchman Nee tried to share with us from his heart in Heart-to-Heart Talks. During his years of ministry he realized that only words coming from a heart that God had spoken to could reach the heart of others. Not that knowledge is unnecessary, but it must come from revelation or enlightenment from above: for only such knowledge is living and operative which can reach the depth of man's heart. Brother Nee wrote and spoke to many hearts to the glory of God.