"He answered and said to them: 'He who sows the good seed is the Son of Man. The field is the world, the good seeds are the sons of the kingdom, but the tares are the sons of the wicked one. The enemy who sowed them is the devil, the harvest is the end of the age, and the reapers are the angels. Therefore as the tares are gathered and burned in the fire, so it will be at the end of this age. The Son of Man will send out His angels, and they will gather out of His kingdom all things that offend, and those who practice lawlessness, and will cast them into the furnace of fire. There will be wailing and gnashing of teeth. Then the righteous will shine forth as the sun in the kingdom of their Father. He who has ears to hear, let him hear!'" (Matt. 13:37-43).
Jesus Christ, the Son of Man, is the one who sows the good seed. There are two classes of people under consideration: the good (represented by wheat) and the wicked (represented by tares). The question is often raised as to whether this parable is talking about wicked men in general or wicked men who are in the church. Both views will be presented briefly.
First, let us consider the view that states that these wicked men are in the church. It is correctly argued that when Jesus talks about "the kingdom", He is usually referring to the church (but not always, as a careful study of Luke 19:11ff illustrates). Thus, many conclude that this parable must be about evil men who are in the church. These evil men in the church are often very hard to distinguish from the righteous (as tares are difficult to distinguish from wheat). The conclusion is then sometimes drawn that since the tares are not to be removed until the harvest, then wicked men in the church should not be removed until the judgment. This conclusion is certainly false for it contradicts several passages of Scripture (e.g., Matt. 18:15-18; I Cor. 5; II Thess. 3:6-15). One could feasibly hold to this first view as long as he does not attempt to prohibit the Scriptural practice of church discipline. It this view is correct, Jesus may be simply issuing a warning against over-zealous efforts to purify the church by violent or overaggressive means. In other words, if sin manifests itself, then it must be dealt with properly, but it is not our duty as Christians to attempt to uncover hidden sins in the lives of our brethren. The Lord will take care of the hypocrites in the church who are quite skilled at pretending to be righteous.
The alternative view states that the wicked men under consideration in this parable are the wicked in general (i.e., both in and out of the church). This view seems more tenable for the simple fact that Jesus plainly declares that "the field is the world" (Matt. 13:38). Jesus didn't say that the field is the church! Thus, Jesus seems to be speaking in a universal sense regarding evil men in the world (cf. 13:24). Therefore, Jesus' main idea would be that His disciples are not to attempt to exterminate evil men. It is not right for a follower of Christ to attempt to "uproot" hypocrites or the ungodly by physical force. God will take care of the wicked in the Judgment!
The harvest is, of course, the Day of Judgment. Those doing the reaping are the angels. They will separate the wicked from the righteous and cast the wicked into the torment of hell. After this, "the righteous will shine forth as the sun" (Matt. 13:43). This is a reference to the heavenly glory that will be experienced by the faithful (cf. Rom. 8:18).
Regardless of which of the above views is correct, we know that God temporarily tolerates the wicked for the sake of the righteous (e.g., Gen. 18:32), and we must sometimes do the same. Followers of Christ today live in a sin-filled world but must strive to remain separate (II Cor. 6:17,18). However, since our knowledge of others is limited, sometimes we may unknowingly fellowship wicked men as brethren being unaware of their lawlessness. May it be understood that God doesn't want His disciples to become professional "tare hunters" for the simple fact that many good brethren would be destroyed in the process. However, when a brother in Christ is clearly not living as one, then the church must discipline him in accordance with the Scriptures.