And he saith unto him, Friend, how camest thou in hither not having a wedding garment? – Matthew 22:12

The wedding garment is simply mentioned here as being a test of loyalty to those who came to the marriage feast, and as a mode by which rebellion was avowed and loyalty made apparent. Here was a man then who came into the gospel feast, and yet refused to comply with the command which related to that feast. He willfully preferred self to God, his heart was full of enmity and pride, he despised the gifts of grace, he scorned the rule of love, he stood a defiant rebel even at the banquet of mercy which his king had spread. His sin lay in coming in there without the wedding garment. If he did not mean to be of one heart with his fellow guests and his lord, why did he come? If a man does not intend to yield himself up to God’s will, why does he profess to be of God’s church? If a man is not saved by the righteousness of Christ, why does he profess to be a believer in Christ? If he will not be obedient to Christ’s holy will, why does he pretend to be follower of Christ? It is a grave mistake for any person to imagine that he can be in the church of God to his own advantage unless his heart is renewed, unless he means what he declares, and sincerely loves the rule under which he professes to put himself.

O my dear hearers, if you have already perpetrated the sin of union with the visible church of God without having the prerequisites, without being indeed submissive to God in heart and desirous to honour Christ, I entreat you, seek what is wanted, seek faith in God, seek a new heart, seek holiness of life, seek to become a loyal subject of the King, and be not content until you have these things, for the King will soon come in: He gives you time as yet, may He also give you grace to see to it that, being now where you ought never to have been, you may yet make your position a right one by obtaining that which will justify you in remaining where you are. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/0976.cfm