"Don’t tear your clothing in your grief, but tear your hearts instead.” Return to the Lord your God, for he is merciful and compassionate, slow to get angry and filled with unfailing love. He is eager to relent and not punish." (Joel 2-13)
In ancient times it was customary to tear one's clothes to express one's dismay, anger or grief. So long as the person concerned was sincere in his or her heart, God recognized it.
However, as is the case with all outward actions, the danger was present that the emotion was insincere or uncontrolled. For this reason the high priest, for example, was forbidden to tear his clothes. The proximity to God, in which he lived, did not permit such an outbreak of human feelings.
Against this background it is easy to understand the meaning of expression "tear your heart". God is teaching us that turning to Him, which cannot be done visibly, is a process that begins to take place deep inside us, in our hearts.
If I regard my heart hitherto and concede that it has led me further and further away from God, towards the abyss, then I must confess and judge my ruined life sincerely before Him.
This will not happen without great concern: one doesn't get over it quickly, and that is precisely what the prophet Joel meant. Once done, the way is open for the second step: "return to the LORD". The two actions are connected: a sincere confession of sins and a decisive rightabout turn. That is repentance, as the Bible teaches.
Repentance does not mean fulfilling certain penances. This practice is unknown in the Bible. Salvation is granted to the believer through God's grace; nobody can earn it.