I never understood this. It always struck me as a bit of vanity. I don't know where this started, either. I can't imagine God cares what you are wearing, and it shouldn't matter to people what you wear as long as it's reasonable. The first Christians were poor people and slaves so I can imagine that they were a dirty bunch wearing rags. But these were Jesus' best friends.
"1 My brothers and sisters, practice your faith in our glorious Lord Jesus Christ by not favoring one person over another. 2 For example, two men come to your worship service. One man is wearing gold rings and fine clothes; the other man, who is poor, is wearing shabby clothes. 3 Suppose you give special attention to the man wearing fine clothes and say to him, “Please have a seat.” But you say to the poor man, “Stand over there,” or “Sit on the floor at my feet.” 4 Aren’t you discriminating against people and using a corrupt standard to make judgments?
5 Listen, my dear brothers and sisters! Didn’t God choose poor people in the world to become rich in faith and to receive the kingdom that he promised to those who love him? 6 Yet, you show no respect to poor people. Don’t rich people oppress you and drag you into court? 7 Don’t they curse the good name of Jesus, the name that was used to bless you?" - 2 James 2:1-7
"1 My brothers and sisters, practice your faith in our glorious Lord Jesus Christ by not favoring one person over another. 2 For example, two men come to your worship service. One man is wearing gold rings and fine clothes; the other man, who is poor, is wearing shabby clothes. 3 Suppose you give special attention to the man wearing fine clothes and say to him, “Please have a seat.” But you say to the poor man, “Stand over there,” or “Sit on the floor at my feet.” 4 Aren’t you discriminating against people and using a corrupt standard to make judgments?
5 Listen, my dear brothers and sisters! Didn’t God choose poor people in the world to become rich in faith and to receive the kingdom that he promised to those who love him? 6 Yet, you show no respect to poor people. Don’t rich people oppress you and drag you into court? 7 Don’t they curse the good name of Jesus, the name that was used to bless you?" - 2 James 2:1-7