What does it mean that a good tree cannot bear bad fruit, and a bad tree cannot bear good fruit?

Jesus is confirming that different trees produce different fruit. Just as one does not find grapes on a thornbush, so you will not find false prophets (bad trees) producing good fruit that is in keeping with new life in Christ.¹ It is possible that thornbushes had fruit which could be mistaken for grapes so that from a certain perspective, what is false appears to be real. The true fruit, however, cannot remain hidden.² Overtime, sooner or later, false prophets will reveal themselves in terms of their behaviour (antinomianism) as well as their teaching (Proverbs 27:22).³