From: Matthew 13:36-43
The Parable of the Weeds Explained
 Then He (Jesus) left the crowds and went into the house. And His disciples came to Him, saying, "Explain to us the parable of the weeds of the field."  He answered, "He who sows the good seed is the Son of Man;  the field is the world, and the good seed means the sons of the Kingdom; the weeds are the sons of the evil one,  and the enemy who sowed them is the devil; the harvest is the close of the age, and the reapers are angels.  Just as the weeds are gathered and burned with fire, so will it be at the close of the age.  The Son of Man will send His angels, and they will gather out of His Kingdom all causes of sin and evildoers,  and throw them out into the furnace of fire; there men will weep and gnash their teeth.  Then the righteous will shine like the sun in the Kingdom of their Father. He who has ears, let him hear."
36-43. While making its way on earth, the Church is composed of good and bad people, just men and sinners: they are mixed in with one another until the harvest time, the end of the world, when the Son of Man, in His capacity as Judge of the living and the dead, will divide the good from the bad at the Last Judgment--the former going to eternal glory, the inheritance of the saints; the latter, to the eternal fire of Hell. Although the just and the sinners are now side by side, the Church has the right and the duty to exclude those who cause scandal, especially those who attack its doctrine and unity; this is can do through ecclesiastical excommunication and other canonical penalties. However, excommunication has a medicinal and pastoral function--to correct those who are obstinate in error, and to protect others from them.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.