A gentile who continues in his impenitent ways after being called out of darkness. Rom. 11:21

The man is you.

St John Chrysostom:

Then in order that not even these should put confidence in their faith alone, He discourses unto them also concerning the judgment to be passed upon wicked actions; to them that have not yet believed, of coming unto Him by faith, and to them that have believed, of care with respect to their life. For the garment is life and practice.

And yet the calling was of grace; wherefore then does He take a strict account? Because although to be called and to be cleansed was of grace, yet, when called and clothed in clean garments, to continue keeping them so, this is of the diligence of them that are called.

The being called was not of merit, but of grace. It was fit therefore to make a return for the grace, and not to show forth such great wickedness after the honor. But I have not enjoyed, one may say, so much advantage as the Jews. Nay, but you have enjoyed far greater benefits. For what things were being prepared for them throughout all their time, these you have received at once, not being worthy. Wherefore Paul also says, And that the Gentiles might glorify God for His mercy. Romans 15:9 For what things were due to them, these you have received.

Wherefore also great is the punishment appointed for them that have been remiss. For as they did despite by not coming, so also thou by thus sitting down with a corrupt life. For to come in with filthy garments is this namely, to depart hence having one's life impure; wherefore also he was speechless.

Do you see how, although the fact was so manifest, He does not punish at once, until he himself, who has sinned, has passed the sentence? For by having nothing to reply he condemned himself, and so is taken away to the unutterable torments.

For do not now, on hearing of darkness, suppose he is punished by this, by sending into a place where there is no light only, but where there is also weeping and gnashing of teeth. Matthew 22:13 And this He says, indicating the intolerable pains.

Hear ye, as many as having partaken of the mysteries, and having been present at the marriage, clothe your souls with filthy deeds. Hear whence you were called.

From the highway. Being what? Lame and halt in soul, which is a much more grievous thing than the mutilation of the body. Reverence the love of Him, who called you, and let no one continue to have filthy garments, but let each of you busy himself about the clothing of your soul.

Hear, you women; hear, you men; we need not these garments that are bespangled with gold, that adorn our outward parts, but those others, that adorn the inward. Whilst we have these former, it is difficult to put on those latter. It is not possible at the same time to deck both soul and body. It is not possible at the same time both to serve mammon, and to obey Christ as we ought.

The entire parable represents God's favor going from one exclusive to the Jews to being broadened to the Gentiles. The Jews are the ones invited first (v3, 5). The servants being the prophets. v7 could represent God's favor being removed from Israel culminating in the destruction of the temple. v8-10 are the gentiles being invited. This is one of the great signs of the destruction of the temple: Matt. 24:14 And this gospel of the kingdom will be proclaimed throughout the whole world as a testimony to all nations, and then the end will come. The end being the end of God dwelling exclusively with Israel.

Given this, the guests are obviously gentiles. The wedding garments could represent a host of "spiritual" clothing. Eph. 6:13-17, Is. 61:10, Job 29:14, Rom. 13:14

The man found to not be wearing these spiritual clothes is trying to communicate that even though you may be at the wedding doesn't mean you won't one day be found out to not belong.

Rom. 11:21 For if God did not spare the natural branches, neither will he spare you.