Jesus tells this parable to an audience that has neither the ears to hear nor the eyes to perceive its truth, and ironically the parable is about why they are like that. A sower goes out to sow and the seed he scatters falls upon four different types of ground, producing four different results, three of which bear no fruit at all.
As I have mentioned earlier, the difference does not lie in the sower's technique of sowing nor in the seed itself, but where the seed falls. Jesus says that the seed is the word of the kingdom, so presumably the sower is anyone who proclaims the word and the ground upon which the seed falls represents the hearts of the hearers. The parable offers no critique of how the sower sows, as if the preacher should be targeting a certain type of hearer or using a certain style of speech or anything of that nature. The seed isn't faulted either, as if the message of the gospel should be adjusted to fit the various types of hearers to maximize its success. By all appearances this parable assumes the faithful, indiscriminate proclamation of the gospel of salvation to anyone who might willingly listen. The lesson is: don't think you can guess the result, because even after you've done your part, the most critical component of this equation lies entirely beyond your control.
When someone hears the word of the kingdom and doesn't understand it, the devil comes and takes away what he has heard. I used to think that the reason the hearer doesn't hear the gospel is because the devil has already taken the word away. But in view of what Jesus said in verse 12 ("whoever does not have, even what he has shall be taken away from him"), a better understanding might be that it is the initial rejection of the gospel that gives Satan the opportunity to remove it further from the heart--so it's the other way around. And God permits Satan to do this as a part of his judgment on the hearer.
Then there are those who hear the word and immediately embrace it with joy. This is scary because it sounds very much like a true conversion. I don't know of any Christian who sees a person embrace the gospel message and immediately thinks, "Hold on, let's not get excited yet. We've got to see whether or not this is rocky soil." Hardly any one of us even considers that possibility. But Jesus warns that many people's understanding of the gospel is only superficial, like shallow roots in rocky soil. Faith never takes root in the soil of their hearts, though it appears to at first, and when persecution arises like a scorching sun these once-enthusiastic types shrivel and fall away. It may appear to be a case of someone "losing their salvation," but actually the problem is that this person had only the appearance of faith. True faith had never taken root in the first place.
In the case of the seed that was choked by thorns, I'm realizing for the first time that there had to be thorns already rooted in this soil. If the soil symbolizes the heart and the thorns symbolize the worries of the world, the problem with this "heart-soil" is that it has embraced both the gospel and the world simultaneously. The seed of the gospel and the seed of worldly desire grow up side by side. But no one can serve two masters, no one can serve both God and money, so the gospel loses out. In such corrupt soil it never really had a chance to begin with.
Some Christians I used to know argued that the thorn-choked seed still represents true believers, it's just that these are "unfruitful Christians." But if Jesus says, "Every tree that does not bear good fruit is cut down and thrown into the fire" (Matthew 7:19), how is there room for these so-called unfruitful Christians in the kingdom? Since Jesus also warned that the gate is narrow that leads to life and the path is broad that leads to destruction, I don't think we should kid ourselves by creating a novel category called the "unfruitful Christian." Jesus asks us to examine our lives for fruit to verify that our faith in genuine. If we have no fruit, then what evidence is there that we truly believe?
The heart of true faith bears fruit like seed producing a crop of a hundredfold, sixtyfold, or thirtyfold. What is this fruit? I don't think this is about merely serving at church or something task-oriented. The fruit of the Spirit is love, joy, peace, patience, kindness, goodness, gentleness, faithfulness and self-control. Most of this fruit has to do with the way you treat other people, especially when you are wronged. It's not saying, "You know you're a Christian when you're patient, kind and good to people who are good to you." It must be talking about having that fruit even with people who are harsh, selfish and manipulative, about having a love that triumphs over every circumstance, that shines the way and brings people to the truth the way you bring in a crop during harvest time.