Tribulation or persecution
refers to the suffering endured by believers for their faith in Jesus (Matthew 24:9, Matthew 24:21, Matthew 24:29; John 16:33; Matthew 5:10, Matthew 5:44; Matthew 10:23; Matthew 23:34). Such suffering separates Jesus’ true followers from hypocrites: those in whom the word does not take root. When trouble comes, they prove unfaithful (not true believers—a term Matthew does not use here).
The persecution and tribulation mentioned in Matthew 13:21 refers to the suffering that believers undergo in this world on account of their faith in Jesus (see Matthew 24:9, Matthew 24:21, Matthew 24:29; John 16:33; and Matthew 5:10, Matthew 5:44; Matthew 10:23; Matthew 23:34). When this happens, the shallow enthusiasts immediately fall away.
Persecution and tribulation separate the true followers of Jesus from the hypocrites—people who receive the word with joy, but in whom it does not go deep enough to take root. When trouble comes, they prove not to be faithful (not true believers—a term Matthew does not use here). They take offense, are led into sin, and ultimately into unfaithfulness.1
20 As for what was sown on rocky ground, this is the one who hears the word and immediately receives it with joy,