How do "once saved, always saved" advocates explain the Parable of the Sower? How do advocates of a strict perseverance of the saints doctrine explain the Parable of the Sower?  That is, how does someone explain it who believes that once someone is saved they cannot lose that salvation?  Jesus' own explanation of the parable says:


  Now the parable is this: The seed is the word of God. The ones along the path are those who have heard; then the devil comes and takes away the word from their hearts, so that they may not believe and be saved. And the ones on the rock are those who, when they hear the word, receive it with joy. But these have no root; they believe for a while, and in time of testing fall away.  And as for what fell among the thorns, they are those who hear, but as they go on their way they are choked by the cares and riches and pleasures of life, and their fruit does not mature. As for that in the good soil, they are those who, hearing the word, hold it fast in an honest and good heart, and bear fruit with patience. (Luke 8:11–15, ESV, emphasis added)


It certainly seems like Jesus is saying some people believe for a while and then stop believing, implying it is possible to give up/lose one's salvation.  How does one resolve this with a "once saved, always saved" theology?  I'm sure the answer will be something along the lines of "they weren't real believers to begin with," but I want more in depth exegesis.  Why are there four groups in the parable if there are only two or three type of people (real believers, "pretend"/apparent believers, unbelievers)?  What do the different groups represent if no group represents people who left the faith (after legitimately having it)?  Why should we take the words of Jesus that some believe, but only for a little while, to mean they didn't really believe at all?

A good answer should reference commentary by a published theologian who adheres to a strict perseverance of the saints doctrine.
As expected, &quot;once saved, always saved&quot; advocates (I'll use &quot;reformed&quot; as shorthand for this) generally agree that both the second and third types of soil represent people who were never saved.  Broadly speaking, they make these points:

The four soils represent four types of people or hearts: the unresponsive, the impulsive, the preoccupied, and the well-prepared
The main difference between the impulsive and the preoccupied is found in the specific reasons that growth does not occur
There are biblical examples of both the impulsive and the preoccupied that we can learn from
Ultimately, the parable should lead us to self-reflection: it should inspire the unbeliever to repent, and the believer to greater faithfulness

The four soils
The breakdown of the four soils given by William Hendriksen is characteristic of reformed commentators:

One might speak of the unresponsive heart (verse 19), the impulsive heart (verses 20, 21), the preoccupied heart (verse 22), and the good, responsive, or well-prepared heart (verse 23). [referring to verses of Matthew 13]1

The statuses of the first and fourth groups are quite obvious, while the second and third are worth further investigation in light of the question: If both of these soils refer to &quot;people who were never saved to begin with,&quot; as reformed commentators believe, what is the point of the distinction? It is to show and warn against two ways that people fail to properly respond to the Gospel.
The impulsive
The profession of faith of the impulsive person, John Gill says, &quot;is taken up in haste, immediately, upon a flash of affection, and a little head knowledge, it does not last long, nor prove honourable.&quot;2 As Hendriksen writes:

The trouble with the people symbolized by seed that fell on rock is that their emotions are superficial, not based on deep-seated convictions.3

The disappearance of this &quot;faith&quot; occurs when trials and persecution come.  But what about the language of Luke 8, where Jesus says that these people &quot;believe for a while&quot;?  Is that not an indication that they have faith, and then lose it?  John Calvin admits that the &quot;honor which they render to the Gospel resembles faith,&quot; and that that they are &quot;widely different from unbelievers, who give no credit to God when he speaks.&quot;  But ultimately, they do not have the &quot;true faith&quot;:

None are partakers of true faith, except those who are sealed with the Spirit of adoption, and who sincerely call on God as their Father; and as that Spirit is never extinguished, so it is impossible that the faith, which he has once engraven on the hearts of the godly, shall pass away or be destroyed.4

John Gill understands them to be the sort of hearers who

whilst they are hearing, assent to what they hear, but when they are gone, either forget it, or, falling into bad company, are prevailed upon to doubt of it, and disbelieve it.5

Biblical examples suggested for this type of person include Simon the magician (Acts 8), the crowds who turn away from Jesus when his teaching is difficult (John 6), those who shouted &quot;Hosanna&quot; and later, presumably, &quot;crucify him&quot; (Mark 11), and those who abandoned the faith in 1 John 2:19.
The preoccupied
The seed in the heart of the preoccupied person, on the other hand, fails to grow not because of trials and persecution, but because of distractions and pleasures.  John MacArthur calls it the &quot;worldly heart, swept up in the deceitfulness of riches.&quot;6 In such a person, Calvin writes, &quot;sinful affections of the flesh prevail,&quot;4 and Hendriksen emphasizes the inability to focus on Christ:

[The preoccupied heart] has no room for calm and earnest meditation on the word of the Lord.  Should any such serious study and reflection nevertheless attempt to gain entrance, it would immediately be choked off.7

Many biblical examples of this type are suggested, including the rich young ruler (Matthew 19), Demas (2 Timothy 4), and the merchants of Amos 8.
Application and summary
Hendriksen writes that the lesson of the parable is:

Examine yourself to discover to which group you belong.  If you belong to any one of the first three groups, be converted!  Not, of course, by power residing in yourself but by God's sovereign grace! Even if you should belong to the fourth group, ask yourself the question, 'Am I sufficiently fruitful?'8

Calvin similarly notes that believers should take heed not to let thorns hinder their fruitfulness:

We ought to labor, no doubt, to pull out the thorns; but as our utmost exertion will never succeed so well, but that there will always be some remaining behind, let each of us endeavor, at least, to deaden them, that they may not hinder the fruit of the word.4

To put it simply: Reformed commentators see the parable as a description of and warning against three distinct types of unbelief, and a call to greater fruitfulness in the Christian walk.

References:

Hendriksen, Exposition of the Gospel According to Matthew, 558.
Gill, Exposition of the Whole Bible, Matthew 13.
Hendriksen, Exposition of the Gospel According to Luke, 427.
Calvin, *Commentary on Matthew, Mark, and Luke, Volume 2.
Gill, Exposition of the Whole Bible, Luke 8.
MacArthur, &quot;Receptivity to the Gospel, Part 3&quot; (sermon).
Hendriksen, Exposition of the Gospel According to Matthew, 561.
Hendriksen, Exposition of the Gospel According to Luke, 429.