The parable of the weeds is an attempt to understand the presence of good and evil in the Church itself.
Along the lines of the parable of the sower, good seed is sown in the field, which is the world at large. But an enemy, that is the evil one, secretly sows weeds into this very field, so that both good and evil are then seen to grow together side by side. Here lies the difficulty – how to account for evil and corruption within the Church at large?
The parable gives the answer: be patient and wait until the final harvest.
The early Church came to recognize that it was not entirely the community of the elect, but there were also unfaithful members present. God tolerates such members in the Church as he tolerates them in the world, but the final judgment would determine their final destiny. The early Church believed that judgment day was imminent, and this Endtime would answer the question satisfactorily.
This parable is a good example of the way in which the early Christian community modified the form and the meaning of the parables, as uttered by Jesus.