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. But when the wheat sprouted and bore grain, then the tares became evident also. The slaves of the landowner came and said to him, 'Sir, did you not sow good seed in your field? How then does it have tares?' And he said to them, 'An enemy has done this!' The slaves said to him, 'Do you want us, then, to go and gather them up?' But he said, 'No; for while you are gathering up the tares, you may uproot the wheat with them. Allow both to grow together until the harvest; and in the time of the harvest I will say to the reapers, "First gather up the tares and bind them in bundles to burn them up; but gather the wheat into my barn."'" [Matthew 13:24-30 NASB]A little later Jesus explains this parable. "Then He left the crowds and went into the house. And His disciples came to Him and said, "Explain to us the parable of the tares of the field." And He said, "The one who sows the good seed is the Son of Man, and the field is the world; and as for the good seed, these are the sons of the kingdom; and the tares are the sons of the evil one; and the enemy who sowed them is the devil, and the harvest is the end of the age; and the reapers are angels. So just as the tares are gathered up and burned with fire, so shall it be at the end of the age. The Son of Man will send forth His angels, and they will gather out of His kingdom all stumbling blocks, and those who commit lawlessness, and will throw them into the furnace of fire; in that place there will be weeping and gnashing of teeth. Then the righteous will shine forth as the sun in the kingdom of their Father. He who has ears, let him hear." [Matthew 13:36-43 NASB]
First, note that the field represents the world. It is not talking specifically about the church in this parable. Although there are applications within the church, it is not the main subject.
The wheat represents true believers in Jesus Christ, and the tares represent those who are not His followers.
Here, Jesus answers the question so many have asked throughout the ages. Why does God allow evil in the world? The answer is: for us.
He has the power to destroy everything and everyone that is evil and ungodly in the world. But He doesn't because He doesn't want to lose any one of us. Peter says, "The Lord is not slow about His promise, as some count slowness, but is patient toward you, not wishing for any to perish but for all to come to repentance." [2 Peter 3:9 NASB]
You see, there is one aspect that this parable doesn't address, which is redemption. At one time we were all tares. But He has the power to turn tares into wheat. We know that "all have sinned and fall short of the glory of God," [Romans 3:23 NASB]. "But God demonstrates His own love toward us, in that while we were yet sinners, Christ died for us." [Romans 5:8 NASB]. Like the wicked ones of the world, we were destined for eternal fire, but He has saved us from the fire, and brought us into His kingdom. "For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord." [Romans 6:23 NASB].
If the Lord swept all evil from the world now, it is possible that many who would have repented of their sins would perish without the opportunity to know Jesus. Thus far, He has delayed His judgement for your sake and mine. He continues to delay judgement for the sake of others.
One day, He will know that the time is complete. All who will come to Him will have come. Then, when the time is right, He will pour out His judgement and wrath upon the earth.
Until then, let us do the work of the kingdom. Let us continue sow His word into the world, that as many as would come to Him would have the opportunity. And instead of moaning about the slowness of His return, let us rejoice that He is taking the time to turn tares into wheat.