“Beware of the scribes, who desire to walk in long robes, and love salutations in the marketplaces, and chief seats in the synagogues, and chief places at feasts,”
His warning is that they beware of a particular type of Scribe of whom there were far too many (not all Scribes could be put on the same level), the showy and ostentatious ones whom everyone noticed, and not be like them. The wearing of long robes was an indication that someone was wealthy enough not to need to work, or it may mainly have in mind special and distinctive festal garments worn on the Sabbath, or the long robes of the teacher. But whichever is in mind (and more than one may be), they were worn in order to draw attention to themselves. We know that special salutations were given to Rabbis, and a certain type of Rabbi loved going through the marketplace so that he would receive the deference that he felt was his due. And they would be offered the chief seats in the synagogues, sitting facing the ordinary worshippers (with the chief one taking ‘Moses' seat' - Matthew 23:2). All this was in order to draw attention to themselves and make them feel good. They loved it. The disciples were to avoid such behaviour, and probably continued to succeed in doing so, but as the centuries went by the so-called Christian leadership would mainly go the way of the Jewish leadership. It is but a short step from deserved distinction to spiritual pride. The pride of life is regularly a huge stumblingblock that stands in the way of those who serve Christ, as it was to the Pharisees and Scribes, and if not checked it eventually produces the worst types of behaviour.
Note how all this apes the picture of the Messiah drawn in the previous passage. Their distinctive clothing, their love of being hailed, their taking of ‘chief seats', their being honoured at feasts, which will be followed by their devouring of widow's houses, is all similar to the behaviour of kings. In their own way they were setting themselves up as messiahs to whom the people should look for deliverance. We are reminded of Paul's words in another context, ‘You have reigned as king's without us, would to God that you did reign' (1 Corinthians 4:8).