Thomas Aquinas,
Summa Theologiae — Question 43 - OF THE MIRACLES WORKED BY CHRIST, IN GENERAL…
“ I answer that, God enables man to work miracles for two reasons. First and principally, in confirmation of the doctrine that a man teaches. For since those things which are of faith surpass human reason, they cannot be proved by human arguments, but need to be proved by the argument of Divine power: so that when a man does works that God alone can do, we may believe that what he says is from God: just as when a man is the bearer of letters sealed with the king's ring, it is to be believed that what they contain expresses the king's will. ”
