Wifey and I went to a concert at a methodist church last night. I noticed little circular holes in the back of the pew along side the shelf where the books are stored. I have VERY vague memories of little glasses of grape juice being there at various times (I haven’t been to regular church service since I was 10).
Did I remember correctly? are those little circular holes, cup holders?
Yep. At least, they were at my church.
Yep. They hold the communion cups after the tray is passed. The alternative is passing the tray and each person takes the cup, drinks and puts the cup back before passing the tray. I prefer the first option.
Pencil holder? So you could write in your contribution or whatever on the envelopes.
Yes, many Protestant churches pass communion juice around in traysfilled with small, thimble-sized glasses. When you are finished, you put the cup in the cupholder.
Yet another confirmation that they’re holders for the tiny cups, based on my experience.
At the Methodist church I attended while growing up in Texas, there were also pencil and envelope holders on the backs of the pews for registration cards, offering envelopes, etc. The “cup holders” are simply through holes drilled in a wooden flange mounted to the back of a pew, and are big enough to stick your finger through - those would not be mistaken for pencil holders.
Whether or not the cup holders were actually used or not varied through the years. Different senior pastors had different ideas for how communion should be done; it became fairly common for communion to be taken up at the front - where there was an entire long railing filled with these same holes, serving the same purpose…
You are all correct about the holes being for communion glasses. I must share a story from my childhood regarding these.
Although now the glasses are plastic, when I was a kid they were made of glass. I was in the choir and sat in the very top of the choir loft (near the point in the sanctuary where all the sound would converge). In those days, the entire congregation would drink the grape juice at the same time, so when they all put those glass containers into the slots, it sounded like a huge waterfall where I was sitting. It was pretty hard not to laugh when this would happen, but we tried to contain that since we were the only ones where that sound was so pronounced.
These days, I think it would be far more useful to have cup holders like you find in a car in the pews. I could find a use for holding a coffee mug every Sunday & communion is only once a month!
During candlelight services, the little cup holders at our church did overtime holding the candles both before they were lit and after extinguishing them.
One of our services acknowledges this sort of caffine dependence - they break after the worship for coffee before the sermon. You know you are in trouble when the preacher specifies double-shot expresso’s only 
In fact another of our services has a coffee machine running at the back, too. That service is held in a pub, but the bar cannot open until after the service. 
Si