It should be obvious that the setting of this parable is not "the marriage supper of the Lamb" when Christ returns and marries His bride (Revelation 19:9), but the preparations for it. God has been sending out the invitations throughout history.
Salvation is a process. Once acceptable for the wedding, God does not judge a person at the doors of the wedding supper. Peter says in I Peter 4:17 that judgment is now on the house of God, spiritual Israel, the church. Revelation 11:18 further shows that Christ will not judge His saints at His return, but is coming to reward the saints and begin the process of judging the nations who have not yet had opportunity at salvation—during the Millennium and the Great White Throne Judgment.
Once a true Christian dies, his judgment is complete. He will either be in the first resurrection and his sins never mentioned to him again, or he will await the third resurrection and death in the Lake of Fire. God does not resurrect him, make him find his way to the wedding supper, and then reject him because he does not have a wedding garment on! If he is qualified for the first resurrection, his salvation is accomplished, and he is automatically part of the bride.
The timing is not of the actual marriage supper, but of a time of calling, of inviting, of evangelism, and even of warning. This parable seems to indicate at least three distinct time frames:
1. When God called a few firstfruits in the Old Testament (see Hebrews 11).
2. Christ's invitation for those who would listen. Most rejected Him, including the leaders of Israel.
3. Those invited by the apostles, continuing to today.
The simple answer to who are the "guests" is that they are the bridal candidates whom the Father has invited wherever and whenever He has seen fit to issue invitations throughout history. Many have been called, informed, invited, offered opportunity, but few are chosen, only 144,000 to be exact. We are invited today to eat at the wedding table—every word of God—but few are responding enough to be chosen. Since "no one can come to Me unless the Father who sent me draws him" (John 6:44), any opportunity for salvation is by special invitation of the Father, automatically putting anyone called in the "guest" category. He must then don wedding garments or be cast out!
After the choosing, sifting, and sorting, God selects a final number of 144,000, and rejects the rest. He will resurrect and change the chosen ones to spirit when Christ returns. At that point, the surviving "guests" or "invited ones" are the bride!
Jesus could not have used the actual bride in the story, for He would have had to include as part of the bride those who had opportunity at salvation and rejected it, and therefore He would cast away "parts" of the bride. What a grisly analogy that would have been! This way, many are invited by analogy as guests, some of whom He can reject and still not reject pieces and parts of the bride.
Christ uses the analogy or figure of guests, but He refers to those who have the potential to comprise the bride. The invitation is no less than to salvation, yet we have seen from other scriptures that only the bride will be part of the first resurrection, so this parable must fit those scriptures as well as make sense as a plausible story.
Remember, this parable is about the Kingdom of God, not an actual wedding feast. Christ is marrying one bride, but she consists of many individuals. So to illustrate His point, He does not refer to the bride as a bride, but as guests. This allows the Father to "throw some out" before the actual wedding.
Herbert Lockyer, in All the Parables of the Bible, says this parable may tie in with I Kings 1:5, 9 and I Chronicles 29:24. These passages describe a pre-wedding feast, common in those days. In ancient Israel such a feast was given at the beginning of a king's reign, who "married" himself to his people. Today, some people do the same kind of thing. They give a pre-wedding dinner for the bridal party followed by a wedding rehearsal.
Who Are the 'Guests at the Wedding'?