That the king came in to see the guests reminds us that we will be subject to an examination in the life hereafter. We will have a proper wedding garment if we are clothed with compassion, goodness, and brotherly love. But there are many who forget that this is necessary and include themselves among assembly, thinking great things of themselves. We have freedom to act in this life, but in the next, if we are not found worthy, we will be bound hands and feet unable any longer to accomplish anything by our own power of action. The guest is cast into the outer darkness. The inner darkness is the darkness of our own hearts, which we can try to remedy in this life, but in the afterlife there will be no remedy for the outer darkness. Many are called - indeed, all are called by God, but the action to accept the call and become one of the chosen is our part. Keep in mind, also, that this parable was spoken for the Jews who were called but were not chosen, since they did not listen.
Does it not seem odd that the king addresses the guest without a wedding garment as friend, despite the guest's transgression? He is a friend because of his faith, but not a friend at the same time because of his lack of love. The guest is silent when the king asks him why he has no wedding garment because he has no excuse. The king himself gave him this garment - in the form of the commandments to love one another and to love God - but somehow the guest lost it before arriving at the marriage feast.