The Catechism of the Catholic Church says, “The religious sense of the Christian people has always found expression in various forms of piety surrounding the Church’s sacramental life, such as the veneration of relics…” (CCC 1674).
In addition to the teachings of Catholic Tradition, relics are clearly mentioned in Sacred Scripture. In Acts of the Apostles, we read this account:
“And God did extraordinary miracles by the hands of Paul, so that handkerchiefs or aprons were carried away from his body to the sick, and diseases left them and the evil spirits came out of them” (Acts 19:11-12).
It is important to note that God is credited with these miracles, not St. Paul. God chose to work these miracles through the relics of a saint.
In addition, Acts 5 says,
“Now many signs and wonders were done among the people by the hands of the apostles. And they were all together in Solomon’s Portico. None of the rest dared join them, but the people held them in high honor. And more than ever believers were added to the Lord, multitudes both of men and women, so that they even carried out the sick into the streets, and laid them on beds and pallets, that as Peter came by at least his shadow might fall on some of them” (Acts 5:12-15).
In this case, they only wanted St. Peter’s shadow to fall on them in order to possibly be healed.
It stands to reason that the saints are associated with these miracles. In John 14:12, Jesus says, “Truly, truly, I say to you, he who believes in me will also do the works that I do; and greater works than these will he do, because I go to the Father.”