The custom among the wealthy at the time of Christ was to provide appropriate cloaks for the guests at a banquet. There would be a cloak room adjacent to the king's banquet hall where each of the guests would vest before entering the party. So, it today's parable, the man without the "wedding garment" was not just someone who had been invited unexpectedly, nor was he too poor to afford the appropriate outfit. The implication was that he simply lacked respect for his hosts and for his fellow guests. That's why the king went so far as to tie him up and throw him out.
But this is a parable, and our Lord taught it to His followers in order to tell them something about the realities of heaven. The king represents God the Father, who had chosen a people for Himself since the time of Abraham. Indeed, Who had promised them a Messiah since the time of Adam and Eve. And when He sent His Son, Jesus Christ, to take His proper place among His people, they rejected Him. They put all but one of His servants, the apostles, to death. So God the Father (the king) called a new group of people to Himself, cutting off many who had been among the descendents of Abraham.
And just like kings did in ancient Palestine, God the Father provided appropriate clothing for His new guests. Through Baptism, he clothed us in the robe of sanctifying grace -- a robe that He expects us to wear all of the time in His presence. He clothed us in a robe of mutual charity and understanding, a robe of holiness and upright behavior. He expects us to wear this robe of holiness all the time, out of respect for Him and out of respect for our fellow guests.
In fact, the relationship that we share goes deeper than any that we can imagine being shared by guests at a banquet. Our Lord tells us in St. John's Gospel that "He is the vine and we are the branches." 1 That's what St. Paul is referring to in today's Epistle: "we are members of one another.2" We are not just individuals; rather, as Christians we are all united to each other in the Mystical Body of Christ. Earlier in the same chapter, he speaks of being called in "one Lord, one Faith, one Baptism, one God and Father of all, who is above all, and throughout all, and in us all."3
Through the Church, Paul tells us that we are no longer "carried about by every wind of doctrine devised in the wickedness of men. Rather, we are to practice the truth in love" and to grow in Christ."4 Yet even doctrinal truth is not enough. He tells us to "put away lying and speak truth each with his neighbor, because we are members of one another."5 It is not enough, in other words, to know the truth about the Trinity or the Resurrection or any of the other tenets of our Faith -- we must also speak the truth to one another, even in the mundane matters of our daily affairs.
"Don't let the sun go down on your anger," let the thief "steal no longer ... let no ill speech proceed from your mouth ... let immorality and obscenity and foolish talk not even be named among you. On the contrary, be kind to one another, and merciful, generously forgiving one another, as God in Christ generously forgives you."6
In the metaphor of the king's banquet, Paul is telling us that we have been invited into the Lord's house and that must be on our best behavior -- respectful to the king, and also to the others whom he has invited. But again, we are more than guests, literally being members of the same Body of Christ.
None of this costs us anything. Indeed, it profits us a great deal. The robe of sanctifying grace comes free with Baptism, and is renewed in each of the Sacraments. Truthfulness and chastity and peaceful behavior are their own reward.
We are members of the Body of Christ, and the invited guests at the marriage supper of the Lamb. Yet, there seems to be a danger that if we choose not to conduct ourselves properly, we may be cast out into the exterior darkness, with weeping and gnashing of teeth." We hear that "many are called but few are chosen," so let us strive to make that choice certain by wearing the garment of Sanctifying grace always, and by "putting on the new man created according to God in justice and holiness of truth."7