We've been going over the book of Esther in Sunday morning class, and there's something I find extremely refreshing about Mordecai. He simply (and all the Jews with him) refused to bow to Haman. Now ask yourself: why does he have to be so stubborn? He could have just bowed a nice little bow, made everyone happy, and said, Oh, it's just a cultural thing. It doesn't change the way we believe. We still have the Gospel. I can go ahead and bow on cue, and not say anything, knowing in my heart that all that matters is the Word.
But he didn't. He refused to do that simple silent thing, which of course made Haman mad, and got Mordecai and his people into one truckload of trouble.
In fact it was a cultural thing. Had he bowed to Haman, his bow would have acknowledged Haman as divine. Here we see that this little liturgical thing, this silent bowing, communicates something. And what it communicated was anathema to Mordecai, so he refused.
This book is written, among other reasons, to exemplify the behavior of Mordecai. We have in the book of Esther a clear and uncompromising exemplification of a simple truth:
When under the title and pretext of external adiaphora such things are proposed as are in principle contrary to God's Word (although painted another color), these are not to be regarded as adiaphora, in which one is free to act as he will, but must be avoided as things prohibited by God. (Formula of Concord, Solid Declaration, X, 5)
See there? How you worship does matter. It did to Mordecai, certainly.