Tares, yet another misguided doctrinal concept in the modern church.
Often people cite Matthew 13:24-30 where Jesus states to leave the tares alone as justification for allowing non-believers in church fellowships, congregations and sanctuaries. Let us look at the full counsel of God's word on the matter to see if this is indeed the case. First here is Matthew 13:24-30 for reference: Another parable put he forth unto them, saying, The kingdom of heaven is likened unto a man which sowed good seed in his field:  But while men slept, his enemy came and sowed tares among the wheat, and went his way.  But when the blade was sprung up, and brought forth fruit, then appeared the tares also.  So the servants of the householder came and said unto him, Sir, didst not thou sow good seed in thy field? from whence then hath it tares?  He said unto them, An enemy hath done this. The servants said unto him, Wilt thou then that we go and gather them up?  But he said, Nay; lest while ye gather up the tares, ye root up also the wheat with them.  Let both grow together until the harvest: and in the time of harvest I will say to the reapers, Gather ye together first the tares, and bind them in bundles to burn them: but gather the wheat into my barn. Matthew 13:24-30 The concept underlying what Jesus was teaching here is that tares are known as counterfeit wheat in a sense. It is because some people look like "believers" on the outside but the inside of their cup, if you will, is unclean. They look like Christians but they are not. If we go about trying to uproot these and toss them out we run the risk of mistakenly uprooting actual believers in the process. But when the "tares" make themselves obvious as an unrepentant sinners we are told to remove them. It is only when you cannot tell the difference that you leave them alone. Moreover if thy brother shall trespass against thee, go and tell him his fault between thee and him alone: if he shall hear thee, thou hast gained thy brother.  But if he will not hear thee, then take with thee one or two more, that in the mouth of two or three witnesses every word may be established.  And if he shall neglect to hear them, tell it unto the church: but if he neglect to hear the church, let him be unto thee as an heathen man and a publican. Matthew 18:15-17 It is reported commonly that there is fornication among you, and such fornication as is not so much as named among the Gentiles, that one should have his father's wife.  And ye are puffed up, and have not rather mourned, that he that hath done this deed might be taken away from among you.  For I verily, as absent in body, but present in spirit, have judged already, as though I were present, concerning him that hath so done this deed,  In the name of our Lord Jesus Christ, when ye are gathered together, and my spirit, with the power of our Lord Jesus Christ,  To deliver such an one unto Satan for the destruction of the flesh, that the spirit may be saved in the day of the Lord Jesus. 1 Corinthians 5:1-5 It is important to understand this because God's design and purpose for His church is that she should only be comprised of genuine believers. We should not allow the obvious unrepentant sinners to remain in the fellowships, congregations and sanctuaries. God calls all of these out specifically so we should not miss what He is commanding us to do. Somehow many miss this command anyways and thus they are being disobedient to God. The adversary hath spread out his hand upon all her pleasant things: for she hath seen that the heathen entered into her sanctuary, whom thou didst command that they should not enter into thy congregation. Lamentations 1:10 I have not sat with vain persons, neither will I go in with dissemblers.  I have hated the congregation of evil doers; and will not sit with the wicked. Psalm 26:4-5 Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?  And what concord hath Christ with Belial? or what part hath he that believeth with an infidel?  And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them ; and I will be their God, and they shall be my people.  Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing ; and I will receive you,  And will be a Father unto you, and ye shall be my sons and daughters, saith the Lord Almighty. 2 Corinthians 6:14-18 But I say, that the things which the Gentiles sacrifice, they sacrifice to devils, and not to God: and I would not that ye should have fellowship with devils.  Ye cannot drink the cup of the Lord, and the cup of devils: ye cannot be partakers of the Lord's table, and of the table of devils. 1 Corinthians 10:20-21 Proving what is acceptable unto the Lord.  And have no fellowship with the unfruitful works of darkness, but rather reprove them. Ephesians 5:10-11 That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ.  And these things write we unto you, that your joy may be full.  This then is the message which we have heard of him, and declare unto you, that God is light, and in him is no darkness at all.  If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth:  But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin. 1 John 1:3-7