Having seen how Jesus fed 5,000 families and then 4,000 families on different occasions, we might think that the disciples would not have any concern about the next meal. Either that or they would always ensure they had sufficient food supplies. So it seems strange to read that they set out on another journey without food because they forgot.
 
The disciples were just thinking about themselves: about what they had to do or should have done. So when Jesus started teaching them about the 'yeast of the Pharisees and Sadducees' all they could think about was the bread they had left behind. Their preoccupation with themselves, their responsibilities and failures had closed their minds to what Jesus was really saying.
 
Even talking about His comment between themselves did not bring fresh light to their understanding: they simply confirmed each other’s original prejudiced view. Their minds were set on earthly things. The more they talked, the firmer their opinion was fixed about what Jesus was saying, even though they were wrong. Divine truth needed divine revelation … which Jesus would supply later (after rebuking their lack of faith).
 
Natural understanding comes from our experiences, but spiritual understanding comes only from God's revelation. Naturally we are resistant to God's Word, preferring our own thoughts and the reassuring collusion of our friends. Our mental habits are so powerfully centred on ourselves that it takes a miracle for God to break through. Even believers who have followed Jesus for many years may find that some areas of life are impervious to His voice in the Scriptures. The only way forward is to confess our self-orientated thinking and ask the Lord to open our spiritual eyes to understand what He is saying (Psalm 119:18).