A catechumen from the Presbyterian persuasion asked me how a handerkchief from St. Paul could have a miraculous effect and why the bones of a saint might have a miraculous effect? He thought there was perhaps some gnostic or scientific answer that could be given to his inquiry. I'd love to know and I'm sure it has nothing to do with matter: as if there were some radioactive residue leaking out of a relic.
Secondary causes. Why not through the heart of St. John Vianney? Pray for my son good St. John Vianney that he might have a Holy vocation to the priesthood and a light unto sainthood.
Sometimes God chooses to work through matter - in Sacraments, sacramentals, statues, medals, religious art, relics of the saints. A good biblical example of this is when Jesus used His own saliva to make mud, which He used to cure the blind man.
God works miracles through the intercession of the saints, and sometimes through their relics, to honor the particular saint, and for us to honor the saint as well. The glory always reflects back to God, and our devotion to the saint honors Him, because it is due to the grace of God that the saint enjoys his or her present status.
I have a first class relic of St. John Vianney. I feel so privileged by God to have in my possession a part of one of His dearest friends. Sometimes, I hold it and pray to St. John, and sometimes I bless myself with it.
In certain ways, the Catholic faith is so concrete, so outward sign-ish, and so very intimate. :o)