This parable helps us to resolve the problem of people who appear to be Christians but after a while it seems that they are not.
Rather than concluding that this problem means it is possible to lose our salvation we are here assured that these people never were true believers in the first place: that their apparent conversion, their apparent following of Jesus Christ, was a ploy of the devil aimed at deceiving both them and genuine Christians.
Satan deceives non-genuine believers into thinking they have come to Christ; when this shallow commitment fails, they, naturally, think that they have 'tried Christianity' and it didn't work. They are then hardened, immunized, against coming to real faith in Christ.
In addition, Satan, by these 'weeds' planted in God' garden, which time reveals as 'weeds' rather than 'wheat', deceives many true Christians into thinking that there is no assurance of salvation, that we can be saved one day and lost the next.
This still leaves the question: what about genuine Christians who are weak in their faith, slow to mature and express their faith in their day to day lives, or going through a rough patch and finding it very difficult to obey God as they ought?
This parable also addresses this problem. The farm workers wanted to pull out all the weeds as soon as they were noticed - just as zealous Christians sometimes want to 'purify' the church. The Master, however, is more gracious, more careful, more understanding. He knows that, just as there are 'weeds' that for a time look like 'wheat', so there is 'wheat' that for a time looks like 'weeds'.
So he says to the zealous servants: 'No ... because while you are pulling the weeds, you may root up the wheat with them. Let them both grow together until the harvest ... The harvest is the end of the age.'
Thus this parable, while presenting a powerful picture of God's judgement, and impacting us with the urgency of making a genuine response to Jesus Christ, also gives us a grand picture of God's grace in which he leaves the reputation of both himself and his visible church open to criticism in his tender compassion for those he has made his own.
As the prophet Isaiah said of the Christ: 'A bruised reed he will not break, and a smoldering wick he will not snuff out.' The eternal salvation of the weakest believer is in the hands, not of our ruthless fellow-believers who are embarrassed by our sin, nor in the hands of any self-protecting, self-promoting pastor who feels threatened by our failures, but in the hands of the tenderest of Shepherds who gathers us in his arms and carries us close to his heart.
Scriptures: Matthew 13:24-30, 36-43; Isaiah 42:3; 40:11.