Jesus presented another parable to them, saying, "The kingdom of heaven may be compared to a man who sowed good seed in his field. But while his men were sleeping, his enemy came and sowed tares among the wheat, and went away. (Matthew 13:24-25)
Most of the mainline denominational churches, as well as, many of the non-denominational churches today in america, are filled with deceived unbelievers; But while his men were sleeping, his enemy came and sowed tares (unbelievers) among the wheat (believers), and went away.
Because many of these churches, even many of the so called "evangelical" churches, are trying to be culturally relevant and acceptable to everyone, they design their services and programs to be entertaining and their preaching to be non-offensive. Little or no mention of sin, judgment and eternal hell. Sound doctrine is seldom preached; For the time will come when they will not endure sound doctrine; but wanting to have their ears tickled, they will accumulate for themselves teachers in accordance to their own desires, and will turn away their ears from the truth and will turn aside to myths. (2 Timothy 4:3-4)
As a result, many unbelievers (tares) are very comfortable blending in and associating with the true believers (wheat). Some of these unbelievers are even in leadership positions in the churches as teachers, elders, deacons and pastors. They are, however, deluded in their unbelief and on the broad road to destruction, not really understanding the gospel and coming judgment; Many will say to Me on that day, "Lord, Lord, did we not prophesy in Your name, and in Your name cast out demons, and in Your name perform many miracles?" And I will declare to them; "I never knew you; Depart from Me, you who practice lawlessness." (Matthew 7:22-23)
Could it be that the most fertile gospel mission fields today are in the churches? Maranatha!