He answered and said unto them, He that soweth the good seed is the Son of man;
Note 1 at Mt 13:37: Jesus made the interpretation of this parable very clear. Satan has secretly placed some of his followers among the church for the purpose of hindering the influence of the church. This has been a more effective strategy than direct opposition. Persecution tends to separate the sheep from the goats (Mt 25:31-33, compare with note 6 at Lu 8:14).
These "wolves in sheep's clothing" (Mt 7:15, 10:16; and Ac 20:29) are very damaging to the body of Christ. However, Jesus warned us against trying to root them out (Mt 13:28-29), especially since it is not always possible to discern other people's hearts. In an effort to destroy these "tares," we might offend some of Christ's "little ones" (Mt 18:6) and cause their profession of faith to waver.
It is important, though, for our own personal benefit that we are aware the children of the wicked one are placed among the true believers. We should be very careful about putting just anyone who professes Christianity into any position of authority (1Ti 3:10). Our best defense is to preach the Word of God without watering it down. False brethren (2Co 11:26 and Ga 2:4) will not endure sound doctrine (2Ti 4:3). They leave when the Word, which is sharper than any two-edged sword (Heb 4:12), begins to expose the thoughts and intents of their hearts (1Jo 2:19).
There will still, however, be those who are deceived and do not recognize that they are not born again, who will remain among the church until the end of this world (Mt 7:21-23 and 13:47-50, see note 1 at Mt 13:31 and note 1 at Mt 13:33). At the second return of Christ, Jesus will send His angels forth to gather all of these "tares" out from among His true church (Mt 13:41) and cast them into a furnace of fire (Mt 13:42).