As Jesus says, the field is the world, in which He has established His church. The church is not of the world (John 17:14), but within it, just as a farmer may designate a specific plot of his land, separated from the rest, for a particular, unique crop.
However, Satan the Devil has also been at work, sowing his own seeds within the field. Using fragments of God's truth, Satan has founded false religions and counterfeit Christianities that preach distortions of truth. Like the tare that grows masquerading as the wheat, members of these false churches may appear good, pious, and very generous. Worldly Christians may possess a seemingly good heart and act with fine intentions, but when the top layer of goodness is peeled back exposing their core, they reveal deceived hearts lacking understanding or true love.
Further, the world's churches are in constant rebellion against God, refusing to keep His commandments and rejecting the absolute authority of His words. The world's ministers even pervert the Word of God with infusions from such pagan religions as Buddhism, Hinduism, or other mystic or New Age faiths. Through syncretism and false doctrine, these churches accomplish the will of their evil father: deceit and destruction (see John 8:44).
Satan's malignant influence is not felt only within the world. He has planted his own seeds, sowing false brethren and even ministers within the very church of God. However, as Christ reveals in this parable, God permits this intrusion of well-camouflaged counterfeits. Tares in God's church will appear religious and devout, with no obvious warning-flag identifying them to unsuspecting church members.