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 13.24-43)
This is a parable, so interpretations may vary, but here are some things I think Jesus is suggesting. Two things in particular.
First, the kingdom is granular rather than an aggregate. The kingdom doesn't exist at scale. You can't point to a group and say, "There is the kingdom of God." Inside that group the kingdom might be found, but not at the level of the group. The group is a mixture of wheat and tares.
Second, this means that the kingdom at the group level will always have a mixed, ambiguous nature. Wheat and tares sit side by side in the pews. To be sure, as Jesus points out, it's not our job to sort this out on earth. Trying to locate and purge the tares is a surefire way to become a tare yourself. Only God will able to sort the situation out.
Perhaps these interpretations are not what Jesus intended. But it does seems clear in the parable that Jesus is suggesting that the kingdom of God is difficult to locate in the world, given that wheat and tares "grow together" side by side. If so, when we look at "the church" we never can see it clearly. "The church" never presents a uniform virtuous face. Rather, "the church" always appears as a mixture of good and bad, as morally ambiguous. That doesn't mean the kingdom doesn't exist. The kingdom exists, rather, on a case by case basis.