Very religious people are, in my experienced , so inward looking that only a religious gift would be suitable . Unfortunately they are usually already surrounded by such gifts. Books are a pretty safe bet when obtained from Christain bookshops .They never sell books that don't show religion in a favourable light.
A lock of Jesus' hair would be very much appreciated, I'm sure!
Seriously, we have a religious (stand-in Minister) family friend and she is sick of getting religious gifts. She would appreciate a donation to a charity the most.