What does Matthew 13:21 mean?
Jesus is describing the meaning of the parable of the sower to His disciples (Matthew 13:1–9), something He did not provide to the crowds to whom He taught the parable (Matthew 13:10–17). In the parable, a sower scatters seed across different kinds of soil. Jesus has shown that those seeds represent the "word of the kingdom," or the good news of the coming kingdom of heaven. Jesus, as the Messiah, is the one who will bring that kingdom to reality.Those represented by rocky soil (Matthew 13:5–6) like the idea of the coming kingdom of heaven. They respond to that news with joy. They seem to immediately understand and accept the truth. Their early growth looks exactly like that found in richer soils. However, persecution and difficult circumstances are like the sun coming out to scorch a plant. Only those with roots providing water and support can survive hardship. Trials expose those whose belief was merely emotional with no real commitment to Jesus. The pressure causes them to quickly fall away from belief in Jesus as the Messiah.
As with all verses in this passage, eternal salvation does not seem to be the main point in question. Jesus' primary lesson here is about how the nation of Israel will respond to His ministry (Matthew 13:10–17). There are clear, useful parallels here to how some people react to the gospel. However, most scholars advise caution about trying to apply this parable as a way of determining whether a particular person is saved.