Back to Reason, and Sensible Veneration
The canonization process was refined over the centuries until, in 1284, papal approval was the only legitimate path to official recognition of sainthood for Roman Catholics. By then lists of saints' names had been compiled, and the days when huge numbers of martyrs were being regularly added to the lists were over.
Relics, too, gave way to more practical symbols. After all, there were only so many bits of bone and hanks of hair to go around, and the church frowned on selling them! Also, the burial sites of saints belonging to the earliest age of the church were uncertain, and saints who were, for example, burned to death sometimes left no relics.
The new symbols that cropped up could be more easily seen by the faithful, or even bought by them. They could be mass-produced as well — or at least as mass-produced as those early centuries would allow. They could be sizable and expensive works of art, or they could be small reminders of a holy person — a drawing, or perhaps a medal or a ring.
Paintings depict saints in a form unique to that person's life, or perhaps death. St. Cecilia, for example, a patron of musicians, is painted playing an organ. St. Peter, the first pope, holds keys, symbols of “the keys of the kingdom.” St. Nicholas is portrayed with three golden balls or bags of gold, symbolizing his gift to the three young sisters without a dowry. And of course there is St. Sebastian with those punishing arrows.