My friend, how is it that you came in here without a wedding garment? If in yesterday's Gospel reading the evangelist Matthew had appeared to have been too harsh and impolite to his predominantly Jewish audience (by telling them that their election does not ipso facto guarantee them a first place in the kingdom), in today's reading he shifts his attention to those members who have now belonged to the kingdom courtesy of the king’s (Jesus) invitation. Those who had found themselves as part of the kingdom through the king’s invitation still had some work to do. As the parable makes it clear, they had to prove that they were worthy of the invitation by embodying the spirit and values of the kingdom. 
Whereas the wedding of a king's son is in itself a public event that is open to all the members of the kingdom (the king’s subject), the wedding reception is often attended by invitation only. The invited guests are presumed to know the requirements of such a high profile event. This should then help us understand the king's action toward the guest who did not don a wedding suit. As a member of the community, he was expected to dress to the occasion for, after all, this was the prince's big day. Even though the guest could have protested that he did not receive the invitation early enough to allow him to have the proper dress (he was “dragged” from the crossroads), once he was in the wedding party, he was a guest and etiquette demanded that he be dressed accordingly. Failure to dress properly was interpreted as a sign of disrespect to the king.
The non-Jewish members of the Matthean community had been outsiders to the faith. But thanks to Jesus, they found themselves as members of a faith community, a faith community that did not require them to observe Jewish traditions. However, there was one thing that they could not be exempted from: putting on the garment of the new faith. By baptism, the doors to the wedding party had been opened to them. But if they were to stay in the party, they had to live up to the demands and ideals of their baptismal call. Lacking this automatically led one to being excommunicated from the community. This is the fate that befell the unfortunate guest. Belonging to this community resulted from responding to an invitation. It is an invitation to which one can respond or fail to respond. But once one has responded, he/she is then expected to put on the garment of the faith.