My dear friend, I do not think that we can tell with any degree of certainty those who truly believe and those who do not. God is the only true judge of men’s souls, because only God can see the hearts of men (1 Samuel 16:7). I do believe for this reason that Christian ministry should include frequent Gospel messages, just in case one with an open heart is in our midst who has never yet believed on Christ Jesus. But now, having said that, I am so glad that you are concerned about the possible presence of “fake” believers in the church, because those who seek to deceive the saints as to their true colors may have very wicked intentions (such as introducing false doctrines or other heresies to the local believers), or they may secretly be living wicked lifestyles, which by association may reflect badly on the whole assembly. In matters concerning fellowship in Christian assemblies, including reception into fellowship, we in the church (true believers) are instructed to guard against association with known evil (2 Timothy 2:16-21). In the Apostle Paul’s first letter to Timothy, he gives this direction as to Christian conduct in the church: “Lay hands suddenly on no man, neither be partaker of other men’s sins: keep thyself pure,” (1 Timothy 5: 22). Today, during this age of the church, it is for true believers in the local assemblies to be careful in reception into fellowship, and to discern and address ungodliness within the church, through assembly discipline, or to separate themselves or remove from fellowship those who practice unrighteousness or who introduce false doctrine.

One Bible reference that clearly deals with this is found throughout the entire chapter of 1 Corinthians 5. Additionally, in 2 Timothy 2:19 we read: “Nevertheless the foundation of God standeth sure, having this seal, The Lord knoweth them that are his. And, Let every one that nameth the name of Christ depart from iniquity.” Thus, we must be careful in our associations so that we are not seen to be in any type of alliance or agreement with ungodliness. And, I do believe there are signs that we can observe such that true believers, through the help of the Holy Spirit, can rightly discern the spirit of ungodliness. Scripture does give us plenty of direction for dealing with such matters. While it is true that we cannot see the heart as God does, certainly we must be careful not to tolerate false doctrine or known sin in our assemblies. The church is charged with dealing with such matters that arise within the local church. As to Scriptural helps for discerning imposters, one Bible portion that comes to mind is where the Lord Jesus told His disciples “…by their fruit ye shall know them,” (Matthew 7:15-20). We are also taught in 1 John 4:1: “Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world,” (see verses 1-6 for the whole thought).

Indeed, in a larger sense, the Kingdom of heaven (which we read about in the parable you cited) is made up of both true believers and also mere professors of faith.  The church, in its universal sense, is in the kingdom of heaven, but not all in the kingdom today are true Christians. Now, in the parable of the wheat and the tares (Matthew 13:24-30;36-43), I believe that the primary application of this is that future time when Christ Himself will return to the earth to execute judgment on all unrighteousness (Acts 17:30,31; Zechariah 14:1-5; Revelation 19 and 20). All true believers will be taken up to be with the Lord prior to this judgment as we see in 1 Thessalonians 4:16-18, but certainly there is some overlap given that as part of the kingdom on earth, the church too will have its imposters. But, again, the Lord knows those that are His, so in the judgment, it is Christ’s work through His angels (not our work) to separate out the wicked for judgment. Again, born again Christians will be taken to glory before this final judgment, but those that are left in Israel whose hearts are true to the Lord will inherit His kingdom on the earth.

In summary, while true believers in Christ today cannot judge the hearts of men, we in the body of Christ must ever be on guard that false doctrine is not introduced into our assemblies, and that known unrighteousness is not tolerated within the church, but is dealt with according to Scripture. In the MacDonald’s Commentary on the Bible, relating to the parable of the wheat and tares, MacDonald writes: “This parable does not justify, as some mistakenly suppose, the toleration of ungodly people in a local Christian church. Remember that the field is the world, not the church. Local churches are explicitly commanded to put out of their fellowship all who are guilty of certain forms of wickedness (1 Cor. 5:9–13). The parable simply teaches that in its mystery form, the kingdom of heaven will include the real and the imitation, the genuine and the counterfeit, and that this condition will continue until the end of the age. Then God’s messengers will separate the false, who will be taken away in judgment, from the true, who will enjoy the glorious reign of Christ on earth.”  (SF)  (571.6)