He put another parable before 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 weeds among the wheat and went away. So when the plants came up and bore grain, then the weeds appeared also. And the servants of the master of the house came and said to him, 'Master, did you not sow good seed in your field? How then does it have weeds?' He said to them, 'An enemy has done this.' So the servants said to him, 'Then do you want us to go and gather them?' But he said, 'No, lest in gathering the weeds you root up the wheat along with them. Let both grow together until the harvest, and at harvest time I will tell the reapers, Gather the weeds first and bind them in bundles to be burned, but gather the wheat into my barn.'" (Matthew 13:24-30 ESV)
Some ministry lessons:
1) There is an enemy -- a real, spiritual enemy -- of God's work. Be sober, stay on the alert. (1 Peter 5:8) Be aware things are not always as they seem: God's children can be immature and sinful; Satan's offspring can appear quite godly and respectable. At any given stage we may not be able to tell the difference between them. Our discernment is not infallible.
2) The church in this age will always be wheat and weeds. They grow together. The great company of Israel, traveling in the wilderness, was a mixed multitude. (Exodus 12:38) We should seek a pure church, but also realize that it is inevitable that good and evil will "grow together" side by side. Here's an amazing thing: the church is not as good as it should be, and the world is not as bad as it could be. (Common grace explains the latter, while remaining sin explains the former.)
3) Take the long view in ministry. Time is the great revealer of natures. How many problems in ministry, seemingly unsolvable and resistant to all my efforts, have over time been sorted out by the Lord. People have been reconciled; children have come back to the Lord; healing has been given. "How long, O Lord?" ...in due time! And some eventually leave the church because they are "not of us" (1 John 2:19).
4) Be careful you don't tear up the good with the bad. Beware prejudging and taking a wholesale approach which damages future growth and potential for change. To merely "do something" may be as bad as to "do nothing." We can do much harm to that which is good while attacking that which is evil.
5) God will sort things out in the end. It is his field and his harvest. He will separate and gather the wheat, and bind and destroy the weeds. We need not fret in the meantime. (Psalm 37)
Be still before the LORD and wait patiently for him;
fret not yourself over the one who prospers in his way,
over the man who carries out evil devices!
Refrain from anger, and forsake wrath!
Fret not yourself; it tends only to evil.
For the evildoers shall be cut off,
but those who wait for the LORD shall inherit the land.
In just a little while, the wicked will be no more;
though you look carefully at his place, he will not be there.
But the meek shall inherit the land
and delight themselves in abundant peace.
(Psalm 37:7-11 ESV)