Can you practice organ in the unheated church during the winter?
Yes, you can, if you have gloves with the fingertips cut off.
Make sure the size of the gloves is on the small side because it will not bother your hands when you play.
Put the woolen socks on too as your feet will freeze dangerously in your organist shoes. Perhaps it's even wiser to play without the shoes but with warm socks in this case.
It goes without saying that you will need a winter coat and a scarf as well.
Also place an electric heater behind the organ bench.
This way you can stay warm and play comfortably for practice, church services or recitals.