To "rend" is an old word meaning TO TEAR.  God uses this word in the scriptures to encourage those who have fallen into complacency and lukewarmness to return to Him in repentance, mourning, and fasting.  And in Joel 2 where God commands His indifferent people to rend their hearts, God also warns of a coming judgment against His people...the kind of judgment that will lead to true repentance.  In today's modern culture of such widespread compromise in the Church, there may also come a time when God will have to come in judgment to awaken His people from their slumber so that we return to loving devotion to Him in purity and with all our hearts.  This teaching is a word in due season for the American church, and for each of us personally.