This parable is to be placed beside the parable of the sower for direct comparison, one extends the other. The first is a look at the process of the individual believer and what he must overcome. The second is a picture of the field (world) of all. The enemy has come in and planted a false believer with every appearance of the true believer except in final fruit. To remove the false believer at this time would also uproot the true believer, so the two are left to grow together. Both parables work together to draw a broader and deeper picture. The Disciples will shortly ask Jesus to explain this further.
This is not to say that the Devil has created his own humans and planted them amongst God's. This is better explained by whom the God created human becomes moral agent of/to. The Kingdom of Heaven becomes firmly planted into one and the false kingdom of heaven becomes established in the other. The differences nearly indistinguishable until the fruit from each is bore out.
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