The veneration of relics is primarily about making a connection between the believer and the core realities of her religion. In this case, a Roman Catholic Christian can see that a great hero of her tradition, St Francis Xavier, actually lived: there are the bones of his hand! It would be as thrilling, and as confirming, as someone viewing what he truly believed were the bones of Moses or the Buddha or Confucius.
Christianity is not just about beliefs, not just about rituals, but is also about a community of believers who help each other along the way. St Francis Xavier was a great helper, and viewing part of his body could reinforce the believer’s sense of truly belonging to a worldwide church of deep history.
“He really existed! It isn’t just a fairy tale!”
At the same time, of course, not everyone will accept the Catholic Church’s promise of a full pardon from purgatory (the “plenary indulgence”), nor believe in what they will take to be more magical parts of this ritual (e.g., that saints’ bodies are supernaturally preserved from decay). And only some Christians will believe that saints can, and should be, prayed to for help.
But all Christians can join in respect for the missionary career of Francis Xavier and rejoice in this opportunity to find out more about his inspiring career of faithful and effective service.