Then he said unto them,
O foolish ones, and slow of heart to believe all that the prophets have spoken:
All Commentaries on Luke 24:25 Go To Luke 24
Cornelius a Lapide
AD 1637
Them said He unto them, 0 fools. Άνόητοι, rendered here in the Vulgate "stulti," but Galatians 3:1., "insensati." With these keen words Christ as the Master rebukes the disciples for their ignorance and slowness to believe. For a teacher is allowed to stimulate his disciples by sharp reproof to the pursuit of higher or more accurate knowledge. See S. Matthew 5:22.
So our nature, frail and dull of understanding, needs some such stimulus to, enable it to believe in spiritual things, and to keep itself steadfast in the hope of their realisation.